Methods and apparatus for obtaining positional information

ABSTRACT

Apparatus for and methods of obtaining positional information about one or more objects in a detection field are disclosed. An array including a transmitting element and a plurality of receiving elements is provided. In one aspect a truncated cross-correlation function is applied to determine the interval between signals received by a plurality of the receiving elements, thereby to determine an angular position of an object. In another aspect a warning zone is defined and it is determined whether an object is within the warning zone. Also disclosed are techniques for stretching received signals, and techniques for obtaining positional information relating to an object using non-Doppler radar. Various implementations, modifications and applications of the techniques described are disclosed. Typical applications of the techniques described are with vehicles.

[0001] The present invention relates to techniques for obtainingpositional information about one or more objects. More specifically, theinvention relates to methods of and apparatus for determining distanceand angular positional information of an object or a plurality ofobjects with respect to a datum.

[0002] Many possible applications of this invention are envisaged. Onespecific field of application is within the automotive industry where asystem embodying the invention might be deployed on a vehicle with afield of detection within or without the vehicle or both. In general, areference to a vehicle in this specification may have particularapplication to a land vehicle, or, more particularly, a motor landvehicle such as a powered road vehicle. The vehicle would usually bewheeled or tracked and/or be incapable of flight.

[0003] For example, there exists a known problem with safety air bagsthat they can cause serious injuries if deployed while a person issitting too close to the bag. Moreover, if the vehicle were not fullyoccupied, it would be desirable to deploy only those bags needed toprotect the occupants. This would minimise the pressure pulse that isgenerated within the vehicle upon deployment and also reduce theconsiderable cost of replacement of the air bags if the vehicle issubsequently repaired. A system embodying the invention might beemployed to determine the number and position of the occupants in avehicle and control the deployment of air bags accordingly; such asystem could apply to any or all of the air bags, whether side or frontair bags. This information could further be used to send an automaticnotification to the emergency services in the event of an accident. Useof the system within a vehicle might be extended to act as an intruderdetection system, to monitor the contents of a load bay or boot, or evento detect movements of the driver characteristic of an onset ofdrowsiness.

[0004] A system embodying the invention could also be employed todetermine the relationship between a vehicle and surrounding objects.For example, a system could provide a driver with a manoeuvring aid toavoid contact with objects or people during reversing or parkingmanoeuvres, and to measure a space to determine whether the vehicle canfit into it. An indication might be provided to warn a driver of thepresence of a vehicle in a mirror blind spot to assist the driver whenchanging lane or merging into traffic. Furthermore, a warning could beissued or the brakes applied to prevent the vehicle from hitting the onein front in slow-moving or congested traffic. Of course, suchapplications are not limited to use on passenger vehicles. They mightequally be applied to commercial vehicles or earth-moving equipment.Such a system can provide an operator of a vehicle with a completeassessment of the situation existing in the vicinity of the vehicle.

[0005] A system embodying the invention might also have aeronauticalapplications, such as collision warning for aircraft, or as a landingaid, particularly for helicopters.

[0006] Further applications of a system embodying the invention mightinclude position monitoring for articles in a handling system such as aproduction line or articles in a post handling system; intruderdetection in a building or an open space as part of a security system;liquid level detection; imaging the interior of a room from outside itswalls and gathering information through walls generally; detecting thepresence of articles within structures such as walls; amongst many otherpossibilities.

[0007] As a further example, there is often a need to determine thelocation of objects, such as cables, pipes or reinforcement rods withinwalls. It is often essential to locate such objects with accuracy priorto carrying out operations on the wall, such as drilling or demolition.A system embodying this invention can be use to provide an accurateindication of the location of such objects.

[0008] A system embodying the invention may also provide data forfurther processing for classifying, tracking and measuring movingobjects.

[0009] U.S. Pat. No. 5,829,782 discloses a system for monitoringoccupancy of a vehicle. The disclosed system operates by emittingsignals within the vehicle and, by means of suitable detectors,monitoring reflections of these waves. The reflected signals areanalysed algorithmically using a variety of pattern-recognitiontechniques to look for the presence of characteristic qualitative and/orquantitative features. The results of the analysis are compared withresults obtained under controlled conditions in order to draw aninference about the occupancy of the vehicle. Since the system operatesby comparing a signal with previously obtained patterns, it does notenable accurate location of arbitrary objects within its detectionfield.

[0010] WO98/00729, proceeding in the name of the present patentapplicant, discloses a radar system, particularly for use on a vehicle,to provide a driver with a warning of potential obstructions in thevicinity of the vehicle. The system establishes a plurality of static,scanning and tracking range gates at which objects can be detected. Thedisclosure of WO98/00729 is hereby incorporated into this specificationby reference.

[0011] WO97/14058, proceeding in the name of the present patentapplicant, discloses a system in which a signal is transmitted into adetection region, and signals reflected from an object in that regionare detected at a plurality of locations. By monitoring thesereflections as the object passes through a plurality of range gates, thetrajectory of the object can be established. While this system is highlyeffective for its intended purpose, it has been found pursuant to thepresent invention that it is not ideally suited to detection of objectsthat are stationary within the detection region. This can limit theeffectiveness of such apparatus in applications of the type describedabove. The disclosure of WO97/14058 is hereby incorporated into thisspecification by reference.

[0012] Angular Resolution by Cross-correlation

[0013] From a first aspect, the invention provides apparatus forobtaining positional information relating to an object, the apparatuscomprising:

[0014] an (optionally fixed) array comprising a transmitting element anda plurality of receiving elements;

[0015] a signal generating stage for applying a series of pulses to thetransmitting element to cause it to transmit a signal, such that atleast a portion of the signal can be reflected from the object to bereceived by the receiving elements;

[0016] a detection stage for detecting signals reflected to thereceiving elements and for generating output signals representative ofthe received signals; and

[0017] a processing stage operable by application of a truncatedcross-correlation function to detect, measure or determine the intervalbetween signals received by a plurality of the receiving elements,whereby to determine an angular position of an object from which thetransmitted signal has been reflected.

[0018] Application of a truncated cross-correlation function to signalsreceived by the array may provide a correlation result that is lessprone to error than is the case for conventional correlation functions.Moreover, the correlation can be achieved more quickly than is the casefor conventional correlation functions. By carrying out thecross-correlation of the received signals in the time domain, the valueof the interval is obtained directly.

[0019] Operation of a system according to this aspect of the inventioncan be contrasted with the operation of a phase comparison monopulseradar system. In such a system, a relationship between phase andfrequency of received signals is first obtained. In many cases, this isdone after mixing with a local oscillator. A delay is then calculatedfrom the relationship between phase and frequency. However, it isrecognised that such systems suffer from intrusion of phase noise thatis inherent in many systems. Moreover, to preserve the unambiguous phaserelationship, it is necessary for two receiving antennas to be placedcloser together than the wavelength of the signal with the result thatgood angular resolution is possible with such systems only by providingseveral such spaced-apart antennas.

[0020] Conventional cross-correlation between two signals includes thestep of summing the products of the two signals over a period (theinterval of correlation) corresponding to the duration of the waveformcontained in the signals. This step is carried out for a series of casesbetween which one of the signals is shifted in time with respect to theother over a range which, in conventional cross-correlation, is alsocomparable with the duration of the waveform.

[0021] In the first aspect of the invention, a truncatedcross-correlation function is applied to the signals received by two ormore of the receiving elements. By truncated cross-correlation it ispreferably meant that the range over which one of the signals is shiftedwith respect to the other is less than that in conventionalcross-correlation. Thus the truncated cross-correlation function ispreferably operable to shift one output signal with respect to anotherover a range which is less than the duration of the signals andpreferably less than the duration of a pulse.

[0022] The maximum time interval between receipt of the same signal attwo receiving elements is equal to the time taken for the signal topropagate directly between them, which is equal to the distance betweenthe elements divided by the speed of propagation of the transmittedsignal. Therefore, it may be appropriate to limit the extent ofcross-correlation to this time value, or to some related value. Thus thetruncated cross-correlation function is preferably operable to shift oneoutput signal with respect to another over a range in which the maximumoffset in either direction is less than 5 times the time that would betaken for the transmitted signal to travel directly from one receivingelement to another, and preferably less than or equal to 3, 2 or 1 timesthis value.

[0023] In this way, the amount by which a signal received by one of thereceiving elements is shifted with respect to a signal received byanother receiving element can be limited to a value corresponding, forexample, to the maximum anticipated delay between the two. This may keepto a minimum the likelihood of false correlation peaks being generatedand reduce the amount of processing required.

[0024] In certain circumstances it may be appropriate to limit themaximum offset to less than the time taken for the transmitted signal totravel directly from one receiving element to another. Such a situationmay arise, for example, if the array has a field of view of less than180°, in which case the maximum offsets may be reduced accordingly. Themaximum positive offset and the maximum negative offset need not beequal, and indeed they may be different, for example if the field ofview is not symmetrical.

[0025] In a typical embodiment, the signal has a characteristicwavelength λ and the interval of correlation (that is the range ofsignals used in the cross-correlation) is a small multiple of λ. Forexample, the interval of correlation may be less than 10, 5, or 2 λ, oreven 1λ. As a minimum, the cross-correlation function is applied overtwo samples, although in many embodiments a greater number of samples isused.

[0026] The interval of correlation may also (or alternatively) berelated to the receiving elements. Specifically, where the receivingelements are spaced apart by a distance D (that is, the distance betweenadjacent receiving elements is D; however, the distance may also varysuch that the D from a first receiving element to a second may bedifferent than the D between either the first or second receivingelement and any other receiving element), the truncatedcross-correlation function may have an interval that is less than asmall multiple of D. For example, the interval of correlation may beless than 5, 2, 1.5 or 1D, or, where the detection field of at least oneelement is less than 180 degrees, it may be 0.9D, 0.8D or 0.7D. This isa relevant consideration because the maximum difference in the length ofthe flight between the same signal received by two receiving elements isthe spacing between those elements.

[0027] Typically the processing stage is operative to identify a maximumvalue of the cross correlation function. The maximum identifies theportions of greatest similarity between the two signals, and, from this,the interval between receipt of the two signals can be deduced.

[0028] Pulses preferably occupy a constant band of frequencies and arepreferably relatively short.

[0029] The first aspect of the invention also provides apparatus forobtaining positional information relating to an object, optionallyincluding any of the above mentioned features, the apparatus comprising:

[0030] transmitting means for transmitting a signal for at least partialreflection by the object;

[0031] a plurality of receiving means for receiving a signal reflectedby the object; and

[0032] processing means for applying a truncated cross-correlationfunction to signals received by a plurality of the receiving means,thereby to determine a position of the object.

[0033] In the first aspect there may also be provided, optionally independence of any apparatus described above, apparatus for obtainingpositional information relating to an object in which the or aprocessing stage is operable to detect the interval between a signalbeing received by a first set of any two or more of the receivingelements and to determine a first angular position of an object fromwhich the transmitted signal has been reflected; and to determine theinterval between a signal being received by a second set (which mayinclude one or more of the elements of the first set) of any two or moreof the receiving elements and to determine a second angular position ofan object from which the transmitted signal has been reflected; theprocessing stage being preferably operable by application of a truncatedcross-correlation function.

[0034] In preferred embodiments, the first and second angular positionsare measured in planes that are substantially not parallel to oneanother. These angular positions can be used to provide two angularvalues of 3-dimensional polar co-ordinates of the object. Mostfavourable resolution of both angles can be achieved if the said planesare approximately normal to one another.

[0035] In apparatus embodying the invention, the processing stage may beoperative to determine the distance from the array of an object fromwhich a signal has been reflected. This can provide the thirdmeasurement required to obtain the co-ordinates of an object inthree-dimensional space.

[0036] Thus, the three-dimensional position of an object may bedetermined with precision by measuring its angular position within whatmight be a relatively broad antenna beam, rather than using a largerantenna or a higher frequency to provide a precisely-tailored, narrowbeam. This may permit the use of devices that have a smallerfrequency/aperture ratio than has hitherto been possible.

[0037] The processing stage may be operable to determine a coordinate inthree-dimensional space of an object from which the transmitted signalhas been reflected, for example by using the angles determined asdescribed in the last two preceding paragraphs.

[0038] In a favoured configuration, at least one of the first and secondset of receiving elements includes three or more elements disposed suchthat that set includes at least two pairs of elements, the spacing ofelements in the two pairs being unequal. This allows the same angularinformation to be obtained from two different pairs of receivingelements, to permit compensation for artefacts of the array, forexample, grating lobes. For example, the spacing (D) between theelements of one pair may be approximately equal to (for example between50% and 200% or between 75 and 150% of) a characteristic wavelength λ ofthe signal, the spacing between a second pair of elements may beapproximately equal to 3λ/4, or 3 D/4, and the ratio of the spacing ofthe elements in one of the first and second pairs to the spacing of theelements in the other of the first and second pairs may be between 0.5and 1 or 0.75 and 0.9. Preferably the relative spacing is arranged sothat grating lobes which arise in the pairs line up differently, thatis, a positive grating lobe for one pair substantially lines up with anegative or zero grating lobe for the other.

[0039] In embodiments according to the last-preceding paragraph, theprocessing stage may be operative to perform a truncatedcross-correlation between the signals received by each pair of elementsand to calculate the product of, or otherwise compare, the results ofthe cross-correlations. This can allow mutual calculation of errors orother artefacts in the received signals.

[0040] Apparatus embodying the invention may further comprise an outputstage operative to generate an output for presentation of positionalinformation relating to the object to a user. For example, the outputmay include at least one of an audible and a visual signal. Thusapparatus embodying the invention can be used to inform a user directlyof the presence of an object in its field of detection.

[0041] Warning Zone

[0042] From a second aspect (which may optionally be provided incombination with the first aspect), the invention provides apparatus forobtaining positional information relating to an object, the apparatuscomprising:

[0043] a warning zone definition stage for defining a warning zone (intwo or three dimensions) within a detection field of the apparatus; and

[0044] a discrimination stage for determining whether a detected objectis within the warning zone;

[0045] in which the warning zone is preferably defined as athree-dimensional region within the detection field.

[0046] This aspect of the invention allows a warning zone to be definedthat is largely independent of the shape of the beam generated by thetransmitting element, that is, the antenna design can be decoupled fromthe zone definition. This can allow the system to operate at frequenciesat which the beam formed by the antenna alone would be too wide, therebyallowing comparatively low frequency (and therefore low cost) apparatusto be used, and to use antennas which are smaller than would otherwisebe applicable, and also less costly.

[0047] Preferably the apparatus further comprises an object locationstage for determining the position of a detected object within thedetection field of the apparatus. The discrimination stage may then beoperable to compare the coordinates of the detected object to thecoordinates of the warning zone to determine whether the object iswithin the warning zone. As will be seen, this arrangement acts todisconnect the function of generating a warning from detection of anobject.

[0048] Typically, the warning zone may be contained within and may besmaller than the detection field of the apparatus. More specifically,the shape of the warning zone may be dissimilar from the shape of thedetection field of the apparatus and/or may be non-circular ornon-spherical. The shape of the warning zone may therefore be tailoredto the needs of a particular application, largely independently of theshape of the detection field.

[0049] For example, the warning zone may include a region (such as aplanar surface) defined in two dimensions within the detection field.

[0050] The warning zone definition stage may include an algorithm thatdefines a warning zone as a function of a coordinate within thedetection field. Such a region may have an essentially arbitrary shapeas defined by the algorithm. Alternatively or additionally, the warningzone definition stage may define at least a limiting value of one ormore ordinates of a coordinate within the detection field. Each suchlimit effectively defines a cut-off of the warning zone in a particulardirection. For example, the warning zone definition stage may define atleast a limiting value of one or more angles of a polar coordinatewithin the detection field. Alternatively or additionally, the warningzone definition stage may define at least a limiting value of a range ofa polar coordinate within the detection field. Such is the essentiallyarbitrary shape of a warning zone, that it may include a plurality ofdiscontinuous spatial regions.

[0051] Apparatus embodying this aspect of the invention may furthercomprise an object location stage for determining the position of adetected object within the detection field of the apparatus. Morespecifically (or alternatively) the discrimination stage may include acoordinate generating stage that generates a coordinate of a detectedobject, which coordinate is then compared with the warning zone. In suchembodiments, the discrimination stage is typically operable to determinethe coordinates of the detected object and compare the determinedcoordinates with the coordinates of the warning zone to determinewhether the object is within the warning zone.

[0052] In embodiments of this aspect of the invention, thediscrimination stage may be operative to generate an output signalindicative that the object is within the warning zone. Apparatusembodying this aspect of the invention may be further operative to issuea warning, for example at least one of an audible, a visual or a tactilewarning to a user upon detection of an object in the warning zone,thereby providing an immediate warning to a user.

[0053] As a development of this aspect of the invention, the warningzone definition stage defines a plurality of warning zones. The warningzones may be non-coextensive (overlapping, separated or spatiallydifferent) and/or alternatively defined, by which it is meant thatdifferent characteristics are used for determining whether an object isin the relevant warning zones. Thus the discrimination stage may beoperable to apply different logic to at least two of the zones. Forexample, different zones may be provided for detecting different speedsor different sizes of objects. This can, for example, be used to providewarnings of multiple levels of severity, depending upon the position orother characteristics of a detected object.

[0054] Apparatus according to the last-preceding paragraph may beoperative to generate an output signal indicative of which of theplurality of warning zones contains the object. For example, it may befurther operative to issue at least one of an audible and a visualwarning to a user upon detection of an object in a warning zone.

[0055] The warning zone may be limited in range and/or may beapproximately cuboid.

[0056] In another development of this aspect of the apparatus, thediscrimination stage is operative to analyse characteristics of objectsoutside of the warning zone. Such characteristics may be, for example,size of the object, distance of the object from the apparatus and/or thewarning zone, direction of movement of the object relative to theapparatus and/or the warning zone, and relative speed of the object. Asan example, the discrimination stage may be operative to track objectsoutside the warning zone and to predict their entry into the warningzone. The apparatus may be operative to issue a pre-warning based on theanalysis.

[0057] For example, if the apparatus is mounted on a vehicle (asdescribed below) in order to provide the driver with a parking aid, theapparatus may issue a pre-warning in the way described above if a largeobject is converging with the vehicle, even though that object may beoutside of the warning zone. This may be particularly desirable, forexample, if the object itself is heading for the vehicle in the samedirection that the vehicle is heading for the object, which may give anincreased risk of collision.

[0058] Apparatus embodying this or any other aspect of the invention maybe suitable for use on a vehicle and may optionally include the vehicle.In such apparatus, at least one of the shape and a relevant dimension ofa warning zone may at least in part be determined in dependence on acorresponding shape and dimension of the vehicle. For example, a warningzone may have a width that is equal to the width of the vehicle plus apredetermined amount (preferably which is less than the width of thevehicle) in order to assist an operator of the vehicle when navigatingnarrow openings. A warning zone may have a height that is equal to theheight of the vehicle (including any accessories, such as a radioantenna) plus a predetermined amount (preferably which is less than theheight of the vehicle), for example to allow for error and forsuspension displacements. A warning zone may have a lower surface thatis substantially planar and spaced above a road surface upon which thevehicle is supported. Spacing the lower surface above the road canexclude false alarms generated by low kerbs, and small, insignificantobjects on the road surface.

[0059] The shape and a relevant dimension of the warning zone ofapparatus for use on a vehicle may be changed in response to vehicleoperating conditions (by the warning zone definition stage). Suchvehicle operating conditions may include at least one of speed,direction of travel, vehicle controls (such as accelerator, brake andclutch position) and ambient environmental conditions (amongst otherpossibilities). For example, the warning zone may extend further forwardfrom the vehicle at high speed, and/or the zone may be extended furtherto one side of the vehicle when cornering.

[0060] Frequency Considerations

[0061] Typical embodiments of the invention in any aspect are operativeto transmit an electromagnetic signal, typically radio frequency, forexample, microwave, radiation. Such radiation is readily controllableand well-suited for the purposes of the present invention.

[0062] For example, the transmitted signal may have a frequency ofbetween 0.5 or 1 and 77 GHz, of between 2 and 25 GHz, of approximatelyone of 0.5 GHz, 1 GHz, 6 GHz, 10 GHz or 2-2.5 GHz. Such signals have auseful ability to penetrate solid objects, including objects made ofwood, plastic material, concrete, brick and other non-metal materials.This might, for example, allow the apparatus to be located on theopposite side of a wall from a region that is to be monitored or withina structure of a vehicle such as a bumper. Moreover, such signals may beused to detect solid metal objects embedded within non-metal objects.These frequency ranges may be implemented using apparatus that is ofadvantageously low cost, and may have an advantageous ability topenetrate solid material. In a typical embodiment, the frequency may beapproximately 2.45 GHz, possibly in order to meet applicable legislativerequirements.

[0063] In general, a particular frequency will be chosen on the basis ofseveral criteria: cost (low frequency is better), size of the antenna(high frequency is better), receiver performance (low frequency isbetter), scattering performance (this depends upon geometry, but a widerelative bandwidth (high bandwidth and low frequency) reduces glint),and applicable legislative requirements.

[0064] The transmitted signal may have a relative bandwidth (say, to thecentre frequency) of approximately 15%, say between 3 and 33%, 5 and 25%or 10 and 20%.

[0065] By virtue of this invention, apparatus operating with a frequencyand/or bandwidth set forth in the preceding paragraphs may be operativeto resolve an angular position of an object with respect to apredetermined datum.

[0066] Pulse Length in Relation to Target/Array

[0067] From a third aspect (which may optionally be provided incombination with either or both preceding aspects or any other aspects),the invention provides apparatus for obtaining positional informationrelating to an object which is operative to transmit a signal into adetection field and to detect a signal reflected from an object in thedetection field, in which the spatial length of the transmitted signalduring its propagation is approximately the same as (say between 50 and200% or 75 and 150%) a dimension of the smallest objects (for example,posts, rails, human limbs, vehicle components and equipment, furniture,aircraft components, etc.) that the apparatus is intended to resolve.This may help to ensure that a reliable correlation can be achievedbetween signals received at different receiving elements.

[0068] In order that the position of objects can be resolved to anaccuracy of several centimetres, the spatial length of the transmittedsignal during its propagation is preferably, in order of magnitude, notgreater than, say, 1.0 m, 0.3 m, 0.1 m, 0.03 m or 0.01 m.

[0069] Two similar, closely spaced objects may give similar returnsignals. If these signals are one half wavelength out of phase with eachother, or thereabouts, the two signals could interfere destructively,resulting in little or no signal. In order to reduce the risk ofdestructive interference, the wavelength of the transmitted signal ispreferably not very much shorter than the spatial length, or, putanother way, each pulse of the transmitted signal may comprise only afew wavelengths. For example, the spatial length of the transmittedsignal during its propagation may be less than 10 wavelengths, or lessthan 6, 5, 3, 2 or even 1 wavelengths, although pulses of longer thanany of these values may also be provided. By restricting the number ofwavelengths per pulse, the likelihood of destructive interferenceoccurring due to two closely spaced objects is reduced.

[0070] If the receiving elements are spaced too far apart thenambiguities may be introduced due to signals arriving at respectivereceiving elements with a delay shift of more than a half wavelength. Onthe other hand, if the receiving elements are too close together theremay be insufficient angular resolution. Thus will be appreciated thatthe choice of distance between receiving elements is a trade off betweenachieving reasonable angular resolution by having a sufficiently largedistance between the receiving elements, and avoiding ambiguity byhaving a sufficiently small distance between the receiving elements. Inpreferred embodiments of the invention, the signal is received at theapparatus at receiving elements spaced by a distance being of the sameorder of magnitude as a characteristic wavelength λ of the signal. Forexample, the receiving elements may be spaced apart by a distance nλwhere n is a real number and 0.5≦n≦10, preferably 1≦n≦5.

[0071] Time Scale Adjustment/Stretching

[0072] In preferred embodiments, the signal generating stage applies aseries of m pulses to the transmitting element to cause it to transmit asignal at times t_(n) where n=1, 2 . . . m, such that at least a portionof the signal can be reflected from the object to be received by thereceiving elements; and

[0073] the detection stage detects a signal reflected to the receivingelements at times r_(n) and generates an output signal representative ofthe received signal;

[0074] wherein the value of r_(n)-t_(n) varies as some function of n.

[0075] By this arrangement, the flight time of a detected signal can bedetermined given only the knowledge of the value of n at which it wasreceived and knowledge of the function of n.

[0076] For example, it may be that the value of r_(n)-t_(n) changeslinearly with n, or it may vary in some other manner, for example in apseudo-random sequence.

[0077] In a typical embodiment, the value of r_(n)-t_(n) increases ordecreases linearly with n, by which it will be understood that the delaybetween a transmit time t_(n) and a corresponding receive time r_(n)increases or decreases linearly with n. Preferably, the delay variesfrom one pulse to an adjacent pulse; this can be a convenient way ofputting the invention into practice. Although in one preferredembodiment the delay varies with each successive pulse, this is notnecessarily the case; a first series of pulses at a first delay may befollowed by a second series of pulses at a second, different, delay, andmore than two different delays may be used. It will be appreciated thatusually the delay is considered to be the delay with respect to the timeat which the relevant pulse is transmitted.

[0078] Successive outputs of the detection stage may be stored in astorage means, the storage means being operable to output a signal ofsubstantially the same shape as the received signal, but with a durationthat is increased in time.

[0079] Preferably, in apparatus embodying the invention, the duration ofeach transmitted signal is less than the interval between transmittedsignals. Most typically, the ratio between the duration of eachtransmitted signal and the mean interval between each transmitted signalis less than 1/10, for example less than 1/20 or 1/50, and greater than1/1000, for example greater than 1/500 or 1/200.

[0080] Preferably, the detection stage is operable to detect thereflected signal during a detection aperture period, which is shorterthan (preferably very much shorter than) the time between successivepulses. In this way, one or more (if a plurality of detection apertureperiods is provided) so-called “range gates” may be provided, asdescribed in WO97/14058; these might typically have widths correspondingto distances of between 1 and 2 cm. With the present invention, however,the range gates would move rather than, as in the prior art, remainstationary.

[0081] A timing stage may be provided to supplying timing signals to thedetection stage and/or the signal generating stage. Preferably, thetiming stage is adapted to operate the detection stage by means of atiming pulse. A timing pulse (or pulses) is an efficient way to generatethe range gate (or gates).

[0082] Preferably, a plurality of spaced receiving elements is provided.With a sufficient number of elements a precise location of the object inspace can be determined. Moreover, if as is preferred the transmittingelements have a wide beam and little individual angular resolution,sufficient elements can be provided so that the angular position ofobjects can be determined by trilateration; that is, by precisepath-length comparison.

[0083] Preferably the apparatus further comprises a computation stagefor processing signals detected by the detection stage, determining thetime interval between a single reflected signal arriving at a pluralityof the receiving elements, and thereby obtaining positional informationrelating to the object from which the signals were reflected.

[0084] The signal generating means preferably operates in an operationcycle to generate a sequence of spaced pulses simultaneously with or ata fixed time after each of a plurality of transmitting trigger instants.The detection stage may be adapted to detect the signals from thereceiving elements simultaneously with or at a fixed time after each ofa plurality of receiving trigger instants, each of which occurs at atime in predetermined relation to the transmitting trigger instants. Forexample, for each transmitting trigger instant t_(n), a correspondingreceiving trigger instant may occur at r_(n), the time intervalr_(n)-t_(n) being a predetermined function of n. In such a system, theinterval r_(n)-t_(n) may be a function of the general form of T₀+nTwhere T₀ and T are constants and n=1, 2 . . . m.

[0085] Preferably the value oft t/T (that is, the ratio between the meaninterval between transmit trigger instants and the increment in thedelay between transmit and receive trigger instants) is of severalorders of magnitude; for example, t/T may be between 10³ and 10⁷, orbetween 10⁴ and 10⁶, typically about 10⁵.

[0086] By arranging the value of r_(n)-t_(n) to increase (or decrease)as a function of n the received signals may be stretched in the timedomain by a factor oft t/T, as will now be explained.

[0087] The output of the detection stage may be fed to a storage means,such as a capacitor or a sample and hold stage, which stores the valueof the signal which is received at each receiving trigger instant r_(n).The signal received at a receiving trigger instant r_(n) may thus bestored until the next receiving trigger instant r_(n+1) (that is, thesignal is sampled and held at each receiving trigger instant) Since eachreceiving trigger instant has a slight increment T in the delay from thecorresponding transmit trigger instant, the value of the signal storedby the storage means changes with each new trigger instant. It thereforetakes t/T repetitions to complete the waveform.

[0088] Assuming that the reflected signal is essentially unchangedbetween trigger instants (such as is the case if the object does notmove significantly relative to the transmitting and receiving elements),then a strobe effect takes place which results in the detected signalbeing stretched by a factor of t/T. In this way the output of thestorage means may be of duration greater than that of the receivedsignals by a factor t/T and of frequency less than that of the receivedsignals by a factor t/T.

[0089] The last-described arrangement can stretch the received signal intime without changing the signal shape. This is beneficial because itcan reduce by a factor of t/T the frequency at which the computationstage can operate. The value of t/T may be considered as a constantdivisor of frequency and multiple of time. This allows processing ofsignals output from the detection stage to take place at a frequencysubstantially less than the frequency of the received signal, with aconsequent advantage in complexity and cost of processing apparatus.

[0090] Thus the apparatus may further comprise a storage means forstoring values of the output signal corresponding to signals received attimes r_(n). The storage means may be operable to output a signal ofsubstantially the same shape as the received signal, but with a durationwhich is increased in time (by a factor of t/T).

[0091] Sampling Stage/Single Control Line

[0092] Apparatus embodying the invention may include a sampling stageoperative under the control of the timing stage selectively to pass orto interrupt the passage of signals from the receiving elements to thedetection stage. The sampling stage may pass signals to the detectionstage for an aperture time t_(a).

[0093] Typically, such apparatus comprises a respective sampling stagefor each receiving element. Advantageously, each sampling stage isconnected to the timing stage by a respective signal delay line, withinwhich delay line a signal is delayed by a time not less than t_(a)/2.This ensures that signals cannot travel from one sampling stage toanother through the timing stage during the aperture time, therebyminimising crosstalk between the sampling stages.

[0094] To facilitate processing of the received signals, the detectionstage typically includes an analogue to digital conversion stage suchthat the output of the detection stage is a digital signal. The outputof the detection stage advantageously includes an indication of theamplitude of the received signals.

[0095] From a further aspect, the invention provides apparatus forobtaining positional information relating to an object according to anypreceding claim, contained within a single housing. Such apparatus maybe hand-holdable in use. For example, the apparatus may be intended toprovide information about the location of objects within a wall.

[0096] Advantageously, apparatus embodying the invention may comprise anantenna array and processing means constructed as a single assembly. Insuch embodiments, the processing means operates to provide allfunctional electrical signals to and receive all functional electricalsignals from the array.

[0097] Apparatus embodying the invention may be intended for use in avehicle.

[0098] Angle Measurement in Vehicle Radar

[0099] Apparatus for obtaining positional information relating to anobject, according to any aspect of the invention, is advantageouslycontained within a single housing. Such apparatus may be particularlyfor use in a land vehicle. This may greatly simplify its installation,for example, in a vehicle. Such apparatus may alternatively behand-holdable for use. This is of particular application in cases wherethe apparatus is embodied in a hand-held tool, such as a device forobtaining information about objects within a wall.

[0100] This feature of the invention may be provided independently.Accordingly there is apparatus for obtaining positional informationrelating to an object which is advantageously contained within a singlehousing. Such apparatus may be particularly for use in a land vehicle.This may greatly simplify its installation, for example, in a vehicle.Such apparatus may alternatively be hand-holdable for use. This is ofparticular application in cases where the apparatus is embodied in ahand-held tool, such as a device for obtaining information about objectswithin a wall.

[0101] From a fifth aspect (which may optionally be provided in anycombination with any other aspect), the invention provides apparatus forobtaining positional information relating to an object, for use on avehicle, for resolving the angular position of an object preferablyusing non-Doppler radar.

[0102] There may also be provided with any aspect of the presentinvention, particularly for a land vehicle, for obtaining positionalinformation relating to an object, apparatus for comprising:

[0103] means for transmitting a probe signal towards the object;

[0104] means for receiving, at a plurality of spaced apart locations,the probe signal as returned by the object; and

[0105] detecting means, coupled to the receiving means, for detectingthe relative timing of the returned probe signals as received at theplurality of locations;

[0106] whereby the positional information for the object can bedetermined from said relative timing.

[0107] Apparatus, particularly for a land vehicle, for obtainingpositional information relating to an object, embodying any combinationof aspects of the invention for use on a vehicle may be for obtainingpositional information relating to an object external of or internal tothe vehicle, the apparatus being operative to generate 3-dimensionalpositional data for the object. This can, for example, provide anoperator of the vehicle with a warning of an obstruction risk, or it mayoverride controls of the vehicle.

[0108] Alternatively or additionally, apparatus embodying anycombination of aspects of the invention for use on a vehicle may be forobtaining positional information relating to an object internal to thevehicle, the apparatus being operative to generate 3-dimensionalpositional data for the object. Typically, such apparatus has adetection field within a passenger compartment of the vehicle.

[0109] In apparatus according to any of the last three precedingparagraphs, the positional data may include at least one of the range,azimuth and elevation of the object.

[0110] Most typically, the antenna array of an embodiment of theinvention for use on a vehicle is carried on a fixed location on thevehicle. Advantageously, the antenna array is located within a componentof the vehicle, preferably a non-metallic component. Thus, visualconflict with the styling of the vehicle can be avoided and the arraycan be protected. For example, the antenna array may be located within abumper (or other portable enclosure) of the vehicle, such as a(preferably a non-metallic) bumper, from which it can generate adetection zone to the front or to the rear of the vehicle.

[0111] In (for example) a vehicle installation, apparatus embodying theinvention may further comprise alerting apparatus for alerting a vehicledriver to the presence of a detected object. Such apparatus may provideto the driver (or another person, or a user of the apparatus, as thecase may be) information that would otherwise be unavailable.

[0112] In such embodiments, the alerting apparatus is operative togenerate an audible warning, which, for example, may include a verbalwarning that may convey information about detected object(s).Alternatively or additionally, the alerting apparatus may be operativeto generate a visual warning. For example, the visual warning mayinclude a visible representation (an image) of the position of an objectdetected by the apparatus.

[0113] Apparatus embodying the invention may further comprise a displayupon which is presented a visual representation e.g. an image of adetection field of the apparatus and an object within the detectionfield.

[0114] Imaging/Pattern Matching

[0115] From a sixth aspect (which may optionally be provided in anycombination with any other aspect), the invention provides apparatus forobtaining positional information relating to an object, for use on avehicle, preferably using non-Doppler radar and being operative todetermine a radar cross-section of an object.

[0116] From a seventh aspect (which may optionally be provided in anycombination with any other aspect), the invention provides apparatus,particularly for use on a (for example land) vehicle, for obtainingpositional information relating to an object, including a transmittingelement for transmitting radiation into a detection field, a receivingelement for receiving radiation reflected from an object in thedetection field, and a processing stage, which is operative to analysethe signals from the receiving element to derive qualitative informationrelating to the object. This permits the apparatus to provide extendeddata relating to objects that it detects.

[0117] For example, the processing stage may be operative to compareinformation relating to an object at successive different angularpositions against a look-up table. This allows variation in the gain ofthe antenna array with angle to be accounted for.

[0118] The processing stage may be operative to determine a radarcross-section of the object. In such embodiments, the processing stagemay be operative to compare the radar cross section with a thresholdvalue of radar cross-section and to generate a warning signal independence upon the result of the comparison.

[0119] Alternatively or additionally, the processing stage may beoperative to determine an evolution of angular position of an objectwith time. In one example, the processing stage is operative todetermine the rate of change of angular position of the object. Inanother example, the way in which the angular position evolves with timeis determined. By determining not just the rate of change of angularposition, but also how the angular position varies with time, theaccuracy with which the relative movement of the vehicle and the objectcan be predicted is increased.

[0120] For example, if the vehicle is moving towards a kerb which isbelow the level of the sensor, the vertical angular position of the kerbwill change as a hyperbola as the vehicle moves towards and over thekerb. By defining the characteristic hyperbola, the movement of thevehicle relative to the kerb can be predicted. In this way, it can bepredicted whether the vehicle will hit the kerb or not, and by how muchthe vehicle will miss the kerb.

[0121] In another example, if the vehicle is moving towards an object,such as a post, which is not directly in the line of travel, then thehorizontal angular position of the object will change as a hyperbola asthe vehicle moves towards and past the object. By defining thecharacteristic hyperbola, it can be predicted whether or not the vehiclewill hit the object, and by how much.

[0122] In a further example, if, as the vehicle moves, a target not onthe centre line maintains a constant angular position as the vehiclemoves, this may indicate a hazard such as a moped which is movingalongside the vehicle at the same speed.

[0123] The above situations may be also be analysed by considering howthe distance away from the object changes as a function of distancemoved by the vehicle. For example, if the distance away from an objectchanges as a straight line against distance moved by the vehicle then itmay be predicted that the vehicle will hit the object, and the point ofcollision may be predicted by extrapolating the line. If the distanceaway from the object changes as a curve against distance moved, then itmay be predicted that the vehicle will miss the object. The amount bywhich the vehicle will miss the object, and the point at which theobject will be closest to the vehicle, may be predicted by estimatingthe evolution of the curve.

[0124] Thus the processing stage may be operative to predict a path ofmovement of the object, preferably relative to the apparatus.

[0125] If the evolution of the angular position of the object differsfrom that which would be produced by a point object, then it may beinferred that the object has an irregular outline, and the outline ofthe object may be predicted based on the difference. For example, if thevehicle is moving towards an object (such as another vehicle) with acurvilinear outline, the radar may see the nearest point of the object,but not the part of the object closest to the path of the vehicle. Bypredicting the outline of the object, it can be predicted where thevehicle will hit the object, even if that part of the object is not yetvisible.

[0126] The invention further provides apparatus for obtaining positionalinformation relating to an object substantially as herein described withreference to the accompanying drawings.

[0127] Applications

[0128] From an eighth aspect, the invention provides a vehicle equippedwith apparatus embodying any of the above-mentioned aspects of theinvention.

[0129] More specifically, the invention may provide a (preferably motor)road vehicle being equipped with a driver warning system comprisingapparatus embodying any of the above-mentioned aspects of the inventionfor obtaining positional information relating to an object external ofthe vehicle, the apparatus being operative to generate 3-dimensionalpositional data for the object. In such a vehicle, the array of theapparatus may be contained within a non-metallic bumper of the vehicle.

[0130] A vehicle embodying this aspect of the invention may comprise adisplay instrument operative to process information obtained by theapparatus and to generate a display therefrom for an operator of thevehicle.

[0131] From a ninth aspect, the invention provides a control system forair bags in a vehicle, the control system comprising apparatus embodyingthe invention, in which the processing stage is operable to determinethe occupancy of a seat equipped with a passenger air bag, and tosuppress deployment of the bag in dependence on the occupancy of theseat. For example, the processing stage may be operable to determinewhether or not the seat is occupied and to suppress deployment of thebag if the seat is unoccupied, and/or to determine if the occupant istoo close to the air bag (such that release of the air bag wouldrepresent a hazard to the occupant) and to suppress deployment of thebag if the occupant is too close.

[0132] This invention also provides embodiments being hand-held toolsand devices for obtaining information about objects within a wallcomprising a apparatus according to any of the preceding aspects of theinvention.

[0133] Antenna Array

[0134] From a tenth aspect, the invention provides an electromagnetic(for example, microwave) antenna array optionally for use in combinationwith any other aspect of the invention, the array including atransmitting element and a plurality of receiving elements, thetransmitting and receiving elements being disposed on a commonsubstrate.

[0135] This feature of the invention may be provided independently.Accordingly the invention provides an electromagnetic (for example,microwave) antenna array, the array including a transmitting element anda plurality of receiving elements, the transmitting and receivingelements being disposed on a common substrate.

[0136] An elecromagnetic antenna array embodying this aspect of theinvention may include a single transmitting element, or a plurality oftransmitting elements.

[0137] An elecromagnetic antenna array embodying this aspect of theinvention may include three (or more) receiving elements arrangednon-collinearly. For example, the receiving elements are arrangedsubstantially at the vertices of a right-angled triangular locus (thatis, in an L-shaped pattern).

[0138] In one embodiment of the invention there is an elecromagneticantenna array optionally for use in apparatus for obtaining positionalinformation relating to an object, the array including a transmittingelement and at least three receiving elements arranged non-collinearly,the transmitting and receiving elements being disposed on a commonsubstrate.

[0139] The receiving elements may be spaced apart by a distance that isthe same order of magnitude as the wavelength λ of the radiation that itis intended to transmit and receive. For example, the receiving elementsmay be spaced apart by a distance mλ where m is less than 10, andpreferably less than 8, 5, 3 or 2, and m is greater than 0.1, andpreferably greater than 0.2, 0.3, or 0.5.

[0140] The efficiency with which the elements can radiate or detectradiation decreases as the size of the elements decreases, since theradiation impedance of the elements may decrease with size, and thus theelements themselves should not be too small. On the other hand, theelements should not be too large, because they may become physically toobig for the array, and because grating lobe effects may occur at largersizes. In general, the size of the elements (whether receiving ortransmitting) is preferably less than 10λ or 4λ and greater than aboutλ/4. In preferred embodiments, the size is in the region of λ/4 or λ/2,although other values may be used. In one particular example, theelements have a size of about 1.5 cm with a wavelength of about 5 cm.

[0141] More advantageously, an elecromagnetic antenna array according tothe present aspect may include four receiving elements arrangednon-collinearly. For example, the receiving elements may be arrangedsubstantially at the vertices of a quadrilateral locus, morespecifically, a trapezoidal or rectangular locus, in which thequadrilateral has long and short parallel sides.

[0142] In accordance with this aspect of the invention theelecromagnetic antenna array may include at least three receivingelements arranged non-collinearly such that there is an axis about whichthe array is asymmetrical.

[0143] This feature may be provided independently. Accordingly there isan elecromagnetic antenna array including at least three receivingelements arranged non-collinearly such that there is an axis about whichthe array is asymmetrical.

[0144] In embodiments where the locus is a trapezium (that is, aquadrilateral having only two sides parallel) , this arrangement canensure that two unequally-spaced pairs of antennas in parallel planescan be selected, with dissimilar artefacts (for example, grating lobes)in their sensitivity patterns. Advantageously, the short side may bebetween 0.5 and 1 times (or approximately three-quarters of) the lengthof the long side. As a specific example, where the trapezial locus haslong and short parallel sides, the length of the shorter side beingapproximately the wavelength λ of the radiation that the array isintended to transmit and receive, and the length of the longer side isapproximately 3λ/2. By suitable processing of signals from such anarray, the effect of grating lobes can be substantially reduced.

[0145] Alternatively, the quadrilateral may have two opposing angleswhich are substantially right angles, while the other two angles are notright angles. This arrangement can ensure that the main grating lobe foreach will point in the correct 3-D direction, while the artefacts willbe different, and thus will cancel out.

[0146] More generally, to achieve the advantages discussed in thelast-preceding paragraph, the invention may provide a microwave antennaarray for use in apparatus for obtaining positional information relatingto an object, which apparatus may be in accordance with any otheraspect, the array including a transmitting element and a plurality ofreceiving elements, in which the spacing of two pairs of (the) receivingelements in a common direction is unequal. This can be achieved in awide range of embodiments, including that discussed above.

[0147] Further Aspects

[0148] From an eleventh aspect, the invention provides anelectromagnetic sensor for use on a vehicle, whether ground-, water- orair-borne, contained in a single enclosure, comprising timing controlmeans, transmitting means, receiving means, processing means andinterface means, in which:

[0149] said transmitting means includes a fixed antenna capable ofemitting a sequence of short pulses of electromagnetic radiation in theultra-high-frequency microwave or millimetre wave band in response totrigger signals from said timing means into a field of view in excess of+/−15 degrees in width and +/−10 degrees in height from a pointingdirection of the antenna,

[0150] said receiving means is responsive to further trigger signalsfrom said timing means and includes one or more fixed receive antennasresponsive to signals corresponding to received fractions of such pulsesafter transmission, propagation to and scattering from one or moreobstacles within such field of view, adapted to convert such signals tosignal information in which each frequency component of such signal isconverted to a respective component of such signal information at afrequency related to such frequency component by a constant divisor forall such respective frequency components,

[0151] said processing means is adapted to acquire said signalinformation thus responsive to said received fractions at each of saidreceive antennas and calculate information concerning said obstacles,

[0152] said interface means are adapted to communicate either to a userof said vehicle or to other electronics systems on the vehicle, which isadapted to measure and provide via such interface means indications ofthe presence or absence of one or more obstacles within a volume whichis not coextensive with the field of view of said antennas and can bedefined independently of such field of view.

[0153] In a sensor according to the last-preceding paragraph theprocessing means may be adapted to provide via such interface meansinformation concerning the distances, azimuth angles and elevationangles from said sensor to said one or more obstacles.

[0154] In a sensor embodying this aspect of the invention, the saidvolume may be contained within one or more surfaces, such as a pluralityof effectively planar surfaces. The processing means may also be adaptedto provide via such interface means indications of the reflectingstrength of the obstacle, proportional to its radar cross section, andindependent of its distance or position in the field of view of thesensor.

[0155] In typical embodiments the antenna array is fixed with respect toits mounting; that is to say, it does not rotate.

[0156] A preferred embodiment of sensor according to this aspect of theinvention is adapted to fit within a bumper (or other portableenclosure) of a vehicle, or within a handheld tool or enclosure.

[0157] From a further aspect, the invention provides apparatus forobtaining positional information relating to one or more objects, theapparatus being operative in an operating cycle for each of m steps inwhich n=1, 2 . . . m, the apparatus including:

[0158] a signal generating stage operative, simultaneously with or at afixed time after a transmitting trigger instant t_(n) to generate asignal, and a transmitting element to transmit said signal into adetection field;

[0159] a plurality of spaced receiving elements operative simultaneouslywith or at a fixed time after a receiving trigger instant r_(n) toreceive a portion of the signal reflected from one or more objects inthe detection field, the interval r_(n)-t_(n) varying as a function of nand having a magnitude in a range corresponding to the times of travelof a signal reflected from an object within the detection field;

[0160] means for identifying the values of n at which signals reflectedfrom one object are received at two or more receiving elements andthereby detecting the time taken, and therefore the distance travelled,by the signals from the transmitting element to the various receivingelements; and

[0161] means for calculating the position of the object from the variouspath lengths thereby identified.

[0162] In an electromagnetic imaging sensor according to this aspect ofthe invention, the transmitting and/or the receiving means are embodiedin a single microchip, with the associated antenna elements printed onone or more adjacent printed circuit cards, and the timing signalgenerator means and control and processing means are embodied in thesame single microchip. The transmitting means may comprise asemiconductor switching device or amplifier external to the said singlemicrochip. Moreover, the receiving means may comprise one or moresemiconductor switching devices or amplifiers external to the singlemicrochip. More specifically, both transmitting and receiving means maycomprise external semiconductor switching devices or amplifiers, and thetiming signal generator and control and processing means are embodied ina single microchip. Yet more specifically, both transmitting andreceiving means comprise semiconductor switching devices or amplifiers,and the timing signal generator and control and processing means may beembodied in two separate microchips.

[0163] Receiving means of embodiments of this aspect of the inventionmay comprise switching samplers in which the switch may be closed for anaperture time less than or comparable with one half the period of saiddominant period. In such embodiments, the switches may all be closed bya common signal without intervening pulse generating circuits.Advantageously, the switching amplifiers may be electrically separatedby lengths of transmission line whose electrical length exceeds or iscomparable to one half the duration of said aperture time.

[0164] The antennas in this aspect of the invention may be stackedmicrostrip patches. Suitably, the stacked microstrip patches may be fedby slots in the circuit card carrying the transmitting means.

[0165] The delay measurement process may, in preferred embodiments,include a cross-correlation process. Such a correlation process may be atruncated correlation process in which the range of the correlation iscalculated between the signals received by any two elements is relatedto the spatial separation of those two elements. For example, the delaymeasurement process comprises measurement of the timing differencebetween comparable features of the waveform such as zero-crossings,peaks, troughs, etc.

[0166] The control and processing means of this aspect of the inventionmay also comprise classification means to identify classes of objectnear to a vehicle by pattern matching with said image.

[0167] The said distance and said angular position of each said objectmay be used to identify the positions of a plurality of said objects intwo or three dimensions. The positions of said plurality of said objectsmay be combined to form an image of the contents of the space near or infront of the sensor.

[0168] In embodiments of this aspect of the invention, the control andprocessing means may comprise or may be connected to further processingmeans in which successive such images or the signals from which theywere generated are used in a synthetic aperture or inverse syntheticaperture process to further detail the image of the contents of thespace near or in front of the sensor. Such an imaging sensor may storedigitally a description of a volume of space near the antenna. Thevolume of space may be other than a sphere or ellipsoid. Moreover, themeasured position of each such object may be compared with such volumeof space near the antenna to determine its location inside or outsidesuch volume.

[0169] The processing means may store a description of the gains of theantennas as a function of solid angle. The measured position of eachobject may be used with such description of the gains to determine thegain of each of the antennas in that direction. In such embodiments (andothers), the measured signal strength arising for each object may bedivided by the product of the antenna gains corresponding to itsposition and multiplied by the fourth power of the measured range toprovide a value proportional to its radar cross-section. For example,the derived value of radar cross-section may be compared with across-section threshold to determine the significance of the target. Thecross-section threshold may be divided by the fourth power of themeasured range and multiplied by the product of the antenna gains in thedirection of the object before comparison with the measured signalstrength.

[0170] Embodiments of the invention also provide apparatus according toany of the above-defmed aspects for generating an image of objectswithin or through a solid object. Typically, the solid object may be awall.

[0171] Further embodiments of the invention provide apparatus accordingto any of the above-defined aspects for providing an image of anenvironment in conditions that human vision is compromised. Forinstance, vision may be compromised by the physiological condition of auser (such as a physical handicap). Alternatively, vision may becompromised by environmental conditions, such as darkness, smoke or fog.

[0172] Method Aspects

[0173] Angular Resolution by Cross-Correlation

[0174] From a first method aspect the invention provides a method forobtaining positional information relating to an object, optionally inapparatus in accordance with any one of the preceding aspects of theinvention, comprising:

[0175] applying a series of pulses to a transmitting element of an arrayto cause it to transmit a signal, such that at least a portion of thesignal is reflected from the object to be received by the receivingelements;

[0176] detecting signals reflected to receiving elements of the arrayand generating output signals representative of the received signals;and

[0177] applying a truncated cross-correlation function to the outputsignals to detect the interval between signals received by a pluralityof the receiving elements, whereby to determine an angular position ofan object from which the transmitted signal has been reflected.

[0178] Preferably the truncated cross-correlation function comprisesshifting one output signal with respect to another over a range which isless than the duration of the signals and preferably less than theduration of a pulse. For example, the truncated cross-correlationfunction may comprise shifting one output signal with respect to anotherover a range in which the maximum offset in either direction is lessthan 5 times the time that would be taken for the transmitted signal totravel directly from one receiving element to another, and preferablyless than or equal to 3, 2 or 1 times this value.

[0179] The signal may have a characteristic wavelength λ and thetruncated cross-correlation function may have an interval of correlationwhich is a small multiple of λ. For example, the interval of correlationmay be less than 10, 5, or 2λ, or even λ. As a minimum, thecross-correlation function is applied over two samples, althoughpreferably a greater number of samples is used.

[0180] In a method embodying this aspect of the invention in which thereceiving elements are spaced apart by a distance D, the truncatedcross-correlation function may have an interval that is less than asmall multiple of D. The interval of correlation may be less than 5, 2,1.5 or 1D.

[0181] Such a method may further include determining the distance fromthe array of an object from which a signal has been reflected. This can,for example, be achieved by multiplying the time taken for the signal tobe received by the speed of propagation of the signal. Typically, amaximum value of the cross correlation function is identified.

[0182] A method embodying the invention may further include a step ofdetermining the distance from the array of an object from which a signalhas been reflected. This may be achieved by multiplying the time takenfor the signal to be received by the speed of propagation of the signal.

[0183] The first method aspect of the invention also provides a methodof obtaining positional information relating to an object, optionallyincluding any of the above mentioned features, the method comprising:

[0184] transmitting a signal for at least partial reflection by theobject;

[0185] receiving signals reflected by the object; and

[0186] applying a truncated cross-correlation function to receivedsignals, thereby to determine a position of the object.

[0187] In the simplest case the signals are received by two receivingelements and the interval between signals received by those tworeceiving elements is determined. If more than two receiving elementsare provided, a truncated cross-correlation function may be applied tosome or all of the various pairs of received signals, such that aplurality of angular positions are determined. In this way the accuracyof the measurement may be improved and/or another dimension added to themeasurement.

[0188] Thus, a method embodying this aspect of the invention may includethe steps of:

[0189] determining the interval between a signal being received by afirst set of any two or more of the receiving elements;

[0190] calculating a first angular position of an object from which thetransmitted signal has been reflected;

[0191] determining the interval between a signal being received by asecond set of any two or more of the receiving elements (which mayinclude one or more of the elements of the first set); and

[0192] calculating a second angular position of an object from which thetransmitted signal has been reflected.

[0193] Preferably, the first and second angular positions are measuredin planes that are substantially not parallel to one another, and moreoptionally, the said planes are approximately normal to one another.

[0194] The method may include determining a coordinate inthree-dimensional space of an object from which the transmitted signalhas been reflected.

[0195] In a method embodying the invention at least one of the first andsecond set of receiving elements includes three or more elements whichmay be disposed such that that set includes at least two pairs ofelements, the spacing of elements in the two pairs being unequal. In anadvantageous embodiment, the spacing (D) between the elements of onepair is approximately equal to (for example between 50% and 200% orbetween 75 and 150% of) a characteristic wavelength λ of the signal, andpreferably the spacing between a second pair of elements isapproximately equal to 3λ/4, or 3 D/4, and preferably the ratio of thespacing of the elements in one of the first and second pairs to thespacing of the elements in the other of the first and second pairs isbetween 0.5 and 1 or 0.75 and 0.9. In such embodiments, a truncatedcross-correlation may be performed between the signals received by eachpair of elements and the product of the result of the cross-correlationsmay be determined.

[0196] A method embodying the invention may further comprise generatingan output for presentation of positional information relating to theobject to a user. The output may include at least one of an audible anda visual signal.

[0197] Warning Zone

[0198] From a second method aspect, the invention provides a method forobtaining positional information relating to an object, optionally incombination with the first aspect, comprising:

[0199] defining a warning zone within a detection field; and

[0200] determining whether a detected object is within the warning zone:

[0201] wherein the warning zone is preferably defined as athree-dimensional region within the detection field..

[0202] Typically, the warning zone is contained within and is smallerthan the detection field. Moreover, the shape of the warning zone may bedissimilar from the shape of (and/or may be smaller than) the detectionfield. Preferably the method includes the step of determining theposition of a detected object within a detection field.

[0203] The warning zone may include a region defined in two dimensionswithin the detection field. For example, the warning zone may be aplanar surface within the detection field. Alternatively, the warningzone may be defined as a three-dimensional region within the detectionfield.

[0204] The warning zone may be defined by an algorithm as a function ofa coordinate within the detection field. Alternatively or additionally,the warning zone may be defined by at least a limiting value of one ormore ordinates of a coordinate within the detection field. For example,the warning zone may be defined by at least a limiting value of one ormore angles of a polar coordinate within the detection field. Moreover,the warning zone may be defined by at least a limiting value of a rangeof a polar coordinate within the detection field. The warning zone mayinclude a plurality of discontinuous spatial regions.

[0205] A preferred method according to this aspect of the inventionfurther includes generation of a co-ordinate of a detected object. Insuch method, the generated co-ordinates maybe compared with co-ordinatesof the warning zone to determine whether the object is within thewarning zone.

[0206] A method embodying the invention may further comprise the step ofgenerating an output signal indicative that the object is within thewarning zone. Typically, the method further comprises the step ofissuing a warning to a user upon detection of an object in the warningzone.

[0207] In a modification to a method embodying the invention, there isdefined a plurality of non-coextensive warning zones. In suchembodiments, there may be included a step of generating an output signalindicative of which of the plurality of warning zones contains theobject.

[0208] In another development of this aspect of the invention the methodmay further comprise the step of analysing a characteristic of an objectoutside of the warning zone. The step of analysing a characteristic maycomprise tracking an object outside the warning zone and predicting itsentry into the warning zone.

[0209] Typical methods embodying the invention further comprise a stepof issuing at least one of an audible and a visual warning to a userupon detection of an object in a warning zone.

[0210] A method embodying this aspect of the invention may be carriedout on a vehicle. In such methods, at least one of the shape and arelevant dimension of a warning zone may be at least in part determinedby a corresponding shape and dimension of the vehicle.

[0211] A method as set forth in the last-preceding paragraph typicallyincludes monitoring operating conditions of the vehicle and changing atleast one of the shape and a relevant dimension of the warning zone inresponse to vehicle operating conditions. Such operating conditions myinclude (amongst other possibilities) at least one of speed, directionof travel, and ambient environmental conditions. For example, thedistance to which the warning zone extends in the direction of travel ofthe vehicle may be increased with the speed of the vehicle.Alternatively or additionally, the extent to which the warning zoneextends to one side of a longitudinal axis of the vehicle may beincreased in a direction in which the vehicle is turning.

[0212] Frequency Considerations

[0213] In a method embodying the invention, the signal is most typicallyan electromagnetic signal. More specifically, in a method embodying theinvention, the electromagnetic signal is most typically microwaveradiation. For example, the electromagnetic signal may have a frequencyof between 0.5 or 1 and 77 GHz, of between 2 and 25 GHz, or it may beapproximately one of 0.5 GHz, 1 GHz, 6 GHz, 10 GHz or 2-2.5 GHz. Forexample, the frequency of the electromagnetic signal may beapproximately 2.45 GHz. The transmitted signal may typically have arelative bandwidth (say, to centre frequency) of approximately 15%,preferably between 3 and 30%, 5 and 25%, or 10 and 20%. The transmittedsignal may have a relative bandwidth to the centre frequency between 10and 20%, between 5 and 25%, between 3 and 33% or approximately 15%.

[0214] In a method embodying any of the last three preceding paragraphs,an angular position of an object may be resolved with respect to apredetermined datum.

[0215] From a third method aspect, the invention provides a method ofobtaining positional information relating to an object, optionally incombination with any of the other method aspects, comprisingtransmitting a signal into a detection field and detecting a signalreflected from an object in the detection field, in which the spatiallength of the transmitted signal during its propagation is approximatelythe same as a dimension of the smallest object that the apparatus isintended to resolve.

[0216] In such a method, the spatial length of the transmitted signalduring its propagation may be, in order of magnitude, not greater thansay, 1.0 m, 0.3 m, 0.1 m, 0.03 m or 0.01 m. For example, the spatiallength of the transmitted signal during its propagation may be less than10 wavelengths, or less than 6, 5, 3, 2 or even 1 wavelengths.

[0217] Typically, in a method embodying this aspect of the invention,the signal is received at receiving elements spaced by a distance beingof the same order of magnitude as a characteristic wavelength λ of thesignal. More specifically, the receiving elements may be spaced apart bya distance nλ where 0.5≦n≦10, or 1≦n≦5.

[0218] Time Scale Adjustment/Stretching

[0219] A method embodying this aspect of the invention may include

[0220] a series of pulses comprises m pulses to cause the transmittingelement to transmit a signal, at times t_(n) where n=1, 2 . . . m; and

[0221] reflected signals are detected by the receiving elements at timesr_(n); comprising the steps of

[0222] generating an output signal representative of the receivedsignal; wherein the value of r_(n)-t_(n) varies as some function of n.

[0223] In preferred embodiments of a method embodying this aspect of theinvention, the value of r_(n)-t_(n) changes linearly with n.Alternatively, it may vary in some other manner, for example in apseudo-random sequence.

[0224] Preferably the method further comprises storing values of theoutput signal corresponding to signals received at times r_(n). Themethod may further comprise outputting a signal of substantially thesame shape as the received signal, but with a duration which isincreased in time. For example, the duration may be increased by severalorders of magnitude, for example, by between 10³ and 10⁷, or between 10⁴and 10⁶.

[0225] Angle Measurement in Vehicle Radar

[0226] From a fourth method aspect, the invention provides a method forobtaining positional information relating to an object, performed on avehicle optionally in combination with any other method aspect of theinvention, for resolving the angular position of an object usingnon-Doppler radar.

[0227] A method embodying the invention may be performed on a vehiclefor obtaining positional information relating to an object external ofthe vehicle, in which 3-dimensional positional data for the object isgenerated.

[0228] A method embodying the invention may be performed on a vehiclefor obtaining positional information relating to an object internal tothe vehicle, in which 3-dimensional positional data for the object isgenerated. In a method embodying the invention, a detection field may bewithin a passenger compartment of the vehicle.

[0229] Positional data obtained by a method embodying the invention mayinclude at least one of the range, azimuth and elevation of the object.

[0230] A method embodying the invention may be carried out by apparatusincluding an antenna array that is carried on a fixed location on thevehicle. Such an antenna array may be located within a (preferablynon-metallic) component of the vehicle. For example, the antenna arraymay be located within a (preferably non-metallic) bumper of the vehicle.

[0231] A method embodying the invention may further comprise a step ofalerting a vehicle driver to the presence of a detected object. In suchembodiments, the alerting step may include generating an audiblewarning. The audible warning may, for example, include a descriptiveverbal warning. Alternatively or additionally, the alerting step mayinclude generating a visual warning. The visual warning may include avisual representation (an image) of the position of detected objects.Such a method may further comprise a step of presenting a visualrepresentation of a detection field and objects within the detectionfield.

[0232] Imaging/Pattern Matching

[0233] From a fifth method aspect, the invention provides a method forobtaining positional information relating to an object, performed on avehicle optionally in accordance with any other method aspect, usingnon-Doppler radar, the method including determining a radarcross-section of a object.

[0234] From a sixth method aspect, the invention provides a method forobtaining positional information relating to an object, optionally inaccordance with any other method aspect, including transmittingradiation into a detection field, receiving radiation reflected from anobject in the detection field, and in an analysis step analysing thesignals from the receiving element to derive qualitative informationrelating to the object.

[0235] In a method embodying this aspect of the invention, a radarcross-section of the object may be determined. The radar cross sectionmay be compared with a threshold value of radar cross-section and awarning signal may be issued in dependence upon the result of thecomparison. Alternatively or additionally, in such a method, anevolution of angular position of an object may be determined, and/or apath of movement of the object may be predicted.

[0236] In the analysis step of a method embodying this aspect of theinvention, the signals may be modified to compensate for angularvariation in sensitivity of the receiving element. Moreover, in theanalysis step the signals may be modified to account for the range ofthe object from which the signals are reflected.

[0237] In a method embodying this aspect of the invention, the analysisstep may include making a comparison between the received signal and apattern corresponding to signals received from a known class of objects.Such an analysis may include identification of characteristic featuresof the received signals. The characteristic features may include atleast one of minima, maxima and zero-crossings.

[0238] This invention also provides a method for obtaining positionalinformation relating to an object substantially as herein described withreference to the accompanying drawings.

[0239] From another aspect, the invention provides a method ofcontrolling deployment of air bags in a vehicle, in which a methodaccording to any other method aspect of the invention is applied todetermine the occupancy of a seat equipped with a passenger air bag, anddeployment of the bag is suppressed in dependence on the occupancy ofthe seat, for example, if the seat is unoccupied or if the occupant istoo close to the air bag.

[0240] A portion of the signal emitted from the transmitting elementwill propagate directly to the receiving elements without beingreflected off an object in the detection field. This will be detected atthe receiving elements a very short time after the transmitting triggerinstant with the result that the ability of the apparatus to resolveobjects at short range may be limited. The receiving elements may bedisposed in a symmetrical relationship with the transmitting elementwhereby such signals will be detected by the receiving elementssubstantially simultaneously and with a substantially similar signalshape. This arrangement can simplify the processing required tocompensate for the existence of these signals. However, in somecircumstances the best results may be obtained if the receiving elementsare not in a symmetrical relationship.

[0241] From another aspect, the invention provides a method of obtainingpositional information relating to an object, preferably in apparatus asaforesaid, the method comprising an operating cycle having m steps inwhich n=1, . . . m, each step comprising:

[0242] (a) generating a signal at a given timing relationship withrespect to a transmitting trigger instant t_(n) and transmitting it intoa detection field; and

[0243] (b) receiving at a given timing relationship with respect to areceiving trigger instant r_(n)at least a portion of the signalreflected from the object;

[0244] wherein the interval r_(n)-t_(n) varies as a function of n.

[0245] Preferably the method further comprises: c) providing at leasttwo (preferably spaced) receiving elements and identifying the values ofn at which signals reflected from the object are received by thereceiving elements. In this way, the time taken, and therefore thedistance travelled, by the signals from the transmitting element to thevarious receiving elements can be determined.

[0246] In this method, step c) typically includes a step ofcross-correlation of the signals received by two of the receivingelements. Preferably the cross-correlation function is a truncatedcross-correlation function and comprises shifting one output signal withrespect to another over a range which is less than the duration of thesignals. Typically one output signal is shifted with respect to anotherover a range in which the maximum offset in either direction is lessthan the time that would be taken for the transmitted signal to traveldirectly from one receiving element to another.

[0247] In order to resolve a three-dimensional position of the object,step c) preferably includes a plurality or even multiplicity of steps ofcross-correlation of the signals received by various of the receivingelements.

[0248] More information about the position and/or the nature of theobject from which the signals were reflected may be obtained byincluding in step c) a comparison of the amplitude of signals receivedby various of the receiving elements. Moreover, step c) may includecomparison of characteristic features of the received signals, suchfeatures including at least one of zero-crossings, maxima and minima.

[0249] In a preferred embodiment of this process, in which thecorrelation coefficient is approximately a cosinusoidal function of eachoffset angle (θ-θ₀), where θ₀ is the angle offset of the object in theplane containing the relevant antenna pair and the antenna boresightdirection), it may not be necessary in step c) to carry out afinely-stepped cross-correlation, but to determine the correlationcoefficients at a small sample of angles, separated by less than halfthe sinusoidal wavelength, to estimate the direction of the maximum,followed by further samples close to that direction to refine theestimate. This further shortens the truncated cross-correlation process.

[0250] After carrying out the truncated cross-correlation process, themethod may include a step of selecting reflections denoted bycorrelation maxima which exceed a predetermined signal threshold (whichmay depend on the noise amplitude observed), and dividing each suchcorrelation maximum by the gain of the transmitting and receivingantennas in the measured direction. Each such correlation maximum maythen be multiplied by the fourth power of the measured range, to obtaina value proportional to the radar cross-section of the object. Thisvalue may then be compared to a cross-section threshold such thatobjects whose cross-section exceeds such threshold are reported or maybe subjected to further processing.

[0251] In a further processing step each three-dimensional positiondetermined in step c) may be compared with a description, either in theform of a look-up table or of an algorithm, which describes a volume ofspace near the antenna. The space may be selected such that the presenceof an object within it is of relevance to a particular situation inwhich the method is being employed. This space will be referred to as“the warning zone”. Objects that are found to be within the warning zoneand which exceed either a signal threshold or a cross-section thresholdmay then be reported, may give rise to a warning, or may be subjected tofurther processing.

[0252] In general, the range of values of r_(n)-t_(n) for 1≦n≦m maycover a range of time within which it is expected that a signalreflected from the object will be received. This can ensure that areflection can be received from an object located anywhere within anintended detection field of the apparatus.

[0253] Further Aspects

[0254] In a preferred apparatus embodying the invention, an array ofelectromagnetic antennas is used in conjunction with processingelectronics in a wideband microwave or millimetre imaging sensorsuitable for obstacle detection, proximity and approach sensing orinspection. Objects in the volume in front of the array are resolved inrange so that substantially only one is found at any one value of range.The range of any such object is determined b the time of flight of apulsed signal reflected from it, and its angular position is thenresolved, as taught in published International patent application no.WO97/14058, by determining the relative times at which such signalsreceived from that object arrive at different elements of the array. Inembodiments of the invention an array is configured with two or moreelements separated by a distance which is less than or comparable withthe dominant wavelength in the radiated pulse; the signals arepre-processed by frequency scaling rather than or in addition tofrequency shifting, and both angular resolution and high detectionperformance are obtained by processing a combination of the channeloutputs according to the element separation to determine timing offsetsand angular positions. Wide bandwidth combined with preciseinter-element receiver timing allows many targets within the range ofthe sensor to be resolved in range and uniquely positioned in angle.

[0255] A method embodying the invention may provide an image of anenvironment in conditions that human vision is compromised. For example,vision may be compromised by the physiological condition of a user (e.g.due to a physical disability). Alternatively or additionally whichvision may be compromised by environmental conditions such as darkness,smoke or fog.

[0256] A method embodying the invention may also be applied to imagingwithin or through a solid object, such as a wall.

[0257] From yet another aspect, there is provided an electromagneticimaging sensor comprising wideband signal transmitting means and aplurality of wideband receiving means, with associated antenna means infixed relative positions within an assembly suitable for attachment toroad vehicles or as portable equipment, timing signal generator means,control and processing means and connection means for connection toalarms, indicators or other systems, in which:

[0258] the transmitting means is operable to emit a train ofelectromagnetic pulses each less than or comparable with 1 nanosecond induration, and characterised by a dominant wavelength or period, butcontaining substantially a small number of cycles of such wavelength orperiod,

[0259] the receiving means and timing signal generator means areoperable to control the transmitting means to emit electromagneticpulses and the receiving means to receive reflections of such pulsesfrom objects in the field of view of the sensor and convert them tosignals in which each frequency component is reduced by a constantfactor, and the time domain waveform is stretched by the inverse of thesame factor, and the arrival time of each component at the receivingelements is measured, being proportional to the distance to the object,

[0260] the pulses are of sufficiently short duration that reflectionsfrom such objects separated by a distance comparable with 0.1 metre ormore in range are substantially resolved in time as they return to thereceiving elements,

[0261] the arrival times of the reflections from each such object ateach receiving element are subjected to a delay measurement process todetermine the relative delays in arrival and therefrom derive thedirection of the arrival from that object, thus determining its angularposition in one, two or three dimensions with respect to the array ofantennas.

[0262] In an electromagnetic imaging sensor according to thelast-preceding paragraph, the transmitting and/or the receiving meansmay be embodied in a single microchip, with the associated antenna feedelements printed on one or more adjacent printed circuit cards. Theantennas themselves may be either part of the cards, or separate fromthem. The timing signal generator means and control and processing meansare preferably embodied in the same single microchip. In suchembodiments, the transmitting means may comprise a semiconductorswitching device or amplifier external to the said microchip. Moreover,the receiving means may comprise one or more semiconductor switchingdevices, amplifiers, filters (which may be adaptive/time-variable) orpulse shaping networks external to the said microchip.

[0263] In one embodiment, both transmitting and receiving means compriseexternal semiconductor switching devices or amplifiers, and the timingsignal generator and control and processing means are embodied in asingle microchip. Alternatively, both transmitting and receiving meanscomprise semiconductor switching devices or amplifiers, and the timingsignal generator and control and processing means are embodied in twoseparate microchips, or on a single chip.

[0264] In embodiments of the invention, the receiving means may compriseswitching samplers in which the switch may be closed for an aperturetime less than or comparable with one half the period of said dominantperiod.

[0265] Most preferably, the receiving means comprise switching samplersin which the switches are all closed by a common signal withoutintervening pulse generating circuits. In such embodiments, theswitching samplers are advantageously electrically separated by lengthsof transmission line whose electrical length exceeds or is comparable toone half the duration of said aperture time.

[0266] In a preferred construction, the antennas are patch antennas suchas microstrip patches. This arrangement has advantages for manufacture.The antennas may be fed by a slotline feed in the circuit card carryingthe transmitting means. Advantageously, the patches may be stackedpatches. This arrangement assists in achieving the bandwidth necessaryfor the antennas.

[0267] The delay measurement process may include a cross-correlationprocess. Advantageously, the cross-correlation process is a truncatedcorrelation process in which the range of the correlation determinedbetween the signals received by any two elements is related to thespatial separation of those two elements. Additionally or alternatively,the delay measurement process comprises measurement of the timingdifference between comparable features of the waveform such aszero-crossings, peaks, troughs, etc..

[0268] The control and processing means may also comprise classificationmeans to identify classes of object near to the sensor by patternmatching with the reflected signals. Thus, the distance and angularposition of each object from which signals are reflected may be used toidentify the positions of a plurality of those objects in two or threedimensions. Thereafter, the positions of said plurality of said objectsmay be combined to form an image of the contents of the space near or infront of the sensor.

[0269] In embodiments of this aspect of the invention, the control andprocessing means may comprise or may be connected to further processingmeans in which successive such images or the signals from which theywere generated are used in a synthetic aperture or inverse syntheticaperture process to further detail the image of the contents of thespace near or in front of the sensor.

[0270] When two objects occur at substantially the same range from theapparatus, a single angular position may be derived correspondingsubstantially to the mean of the positions of the objects weighted bythe amplitudes of said reflections of said pulses.

[0271] In a related aspect, the invention provides apparatus forobtaining positional information relating to one (or more) object(s),the apparatus comprising:

[0272] an array including a transmitting element and a receiving element(preferably a plurality of spaced receiving elements);

[0273] signal generating means for applying (operable to apply) a seriesof pulses to the transmitting element to cause it to transmit a signal,such that at least a portion of the signal can be reflected from theobject to be received by the receiving element;

[0274] detection means for detecting (operable to detect) a signalreflected to the receiving element (during a detection aperture period)and for generating (operable to generate) an output signalrepresentative of the received signal; and

[0275] timing means for operating the detection means at a varyingdelay.

[0276] The timing means may be adapted or operable to initiate operationof the detection means after a variable interval following eachoperation of the signal generating means, the said interval varying withsuccessive pulses.

[0277] If a plurality of receiving elements is provided, the computationmeans may be adapted or operable to process signals detected by thedetection means, assess the time interval between a single reflectedsignal arriving at two or more of the receiving elements, and therebydetermine the position of the object from which the signals werereflected.

[0278] In a further related aspect, the invention also providesapparatus for obtaining positional information relating to an object,the apparatus being operative in an operating cycle having m steps inwhich n=1, . . . m, the apparatus comprising:

[0279] a signal generating stage for generating a signal at a giventiming relationship with respect to a transmitting trigger instantt_(n);

[0280] a transmitting element to transmit the said signal into adetection field; and

[0281] a receiving element for receiving at least a portion of thesignal reflected from the object at a given timing relationship withrespect to a receiving trigger instant r_(n);

[0282] wherein the interval r_(n)-t_(n) varies as a function of n.

[0283] The invention may also extend to apparatus for obtainingpositional information relating to one or more objects, the apparatuscomprising:

[0284] an array including a transmitting element and a plurality ofspaced receiving elements;

[0285] signal generating means operative to apply a series of pulses tothe transmitting element to cause it to transmit a signal, such that aportion of the signal can be reflected from one or more objects to bereceived by the receiving elements;

[0286] detection means operative to detect signals reflected to thereceiving elements during a detection aperture period and to generate anoutput signal representative of the received signals;

[0287] timing means operative to initiate operation of the detectionmeans after a variable interval following each operation of the signalgenerating means, the said interval varying with successive pulses;

[0288] computation means operative to process signals detected by thedetection means, assess the time interval between a single reflectedsignal arriving at two or more of the receiving elements, and therebydetermine the position of the object from which the signals werereflected.

[0289] The invention may further extend to apparatus for obtainingpositional information relating to one or more objects, the apparatusbeing operative in an operating cycle for each of m steps in which n=1,2 . . . m, the apparatus including:

[0290] a signal generating stage operative, simultaneously with or at afixed time after a transmitting trigger instant t_(n) to generate asignal, and a transmitting element to transmit the said signal into adetection field;

[0291] plurality of spaced receiving elements operative simultaneouslywith or at a fixed time after a receiving trigger instant r_(n) toreceive a portion of the signal reflected from one or more objects inthe detection field, the interval r_(n)-t_(n) varying as a function of nand having a magnitude in a range corresponding to the times of travelof a signal reflected from an object within the detection field;

[0292] means for identifying the values of n at which signals reflectedfrom one object received at two or more receiving elements and therebyassessing the time taken, and therefore the distance travelled, by thesignals from the transmitting element to the various receiving elements;and

[0293] means for calculating the position of the object from the variouspath lengths thereby identified.

[0294] The invention additionally provides apparatus for generating animage of objects within or through a solid object in accordance with anyof the preceding aspects of the invention. In such cases, the solidobject is a typically a wall.

[0295] The invention further provides apparatus in accordance with anyof the preceding aspects of the invention for providing an image of anenvironment in conditions that human vision is compromised. For example,in such embodiments, vision may be compromised by the physiologicalcondition of a user. Alternatively or additionally, vision might becompromised by environmental conditions.

[0296] In addition to all of the above, the invention provides apparatusfor obtaining positional information relating to an object, comprising:transmitting means for transmitting a probe signal towards the object,the transmitting means comprising: a signal generating stage; and atleast one transmitting element; receiving means for receiving, at aplurality of spaced apart locations, the probe signal as returned by theobject, the receiving means comprising; at least one receiving elementat the plurality of spaced apart locations; and detecting means fordetecting the relative timing of the returned probe signals as receivedat the plurality of locations, the detecting means comprising; adetection stage, coupled to the receiving means; whereby positionalinformation for the object can be determined from the relative timing;and wherein: the signal generating stage applies a series of m pulses tothe transmitting element to cause it to transmit a signal, at timest_(n) where n=1, 2 . . . m, such that at least a portion of the signalcan be reflected from the object to be received by the receivingelements; the detection stage detects a signal reflected to thereceiving elements at times r_(n) and generates an output signalrepresentative of the received signal; and wherein the value ofr_(n)-t_(n) varies as some function of n.

[0297] The invention provides apparatus for obtaining positionalinformation relating to an object, comprising: means for transmitting aprobe signal towards the object, said transmitting means comprising atransmitting element; means for receiving, at a plurality of spacedapart locations, the probe signal as returned by the object, saidreceiving means comprising a plurality of receiving elements; anddetecting means, coupled to the receiving means, for detecting therelative timing of the returned probe signals as received at theplurality of spaced apart locations; whereby the positional informationfor the object can be determined from said relative timing; and whereinthe transmitting element and receiving elements are disposed on a commonsubstrate.

[0298] And it provides an antenna array optionally for use in apparatusfor obtaining positional information relating to an object embodying oneor more aspects of the invention, the array including a transmittingelement and a plurality of receiving elements, the transmitting andreceiving elements being disposed on a common substrate.

[0299] The invention further provides a method of obtaining positionalinformation relating to an object, comprising the steps of: transmittinga probe signal towards the object; receiving, at a plurality of spacedapart locations, the probe signal as returned by the object; detectingthe relative timing of the returned probe signals as received at theplurality of locations; and determining positional information for theobject from the relative timing; wherein: the transmitting stepcomprises applying a series of m pulses to a transmitting element tocause it to transmit a signal, at times t_(n) where n=1, 2 . . . m, suchthat at least a portion of the signal can be reflected from the objectto be received at the plurality of spaced apart locations; the detectingstep comprises detecting a signal reflected to the receiving elements attimes r_(n)and generating an output signal representative of thereceived signal; and wherein the value of r_(n)-t_(n) varies as somefunction of n.

[0300] The invention also provides a method of obtaining positionalinformation relating to an object using an apparatus comprising atransmitting element, a receiving means comprising a plurality ofreceiving elements and a detecting means, the method comprising:transmitting a probe signal from the transmitting element towards theobject; receiving, at a plurality of spaced apart locations, the probesignal as returned by the object; and detecting, at the detecting means,the relative timing of the returned probe signals as received at theplurality of spaced apart locations; determining the positionalinformation for the object from said relative timing; wherein thedetecting means is coupled to the receiving means and the transmittingelement and receiving elements are disposed on a common substrate.

[0301] The invention further provides the use of an electromagneticantenna array in a method described above in which the receivingelements are spaced apart by a distance that is the same order ofmagnitude as the wavelength λ of the radiation that it is intended totransmit and receive, the electromagnetic antenna array including atleast three receiving elements arranged non-collinearly such that thereis an axis about which the array is asymmetrical.

[0302] The invention also provides the use of an electromagnetic antennaarray in a method of obtaining positional information relating to anobject, the array including a transmitting element and at least threereceiving elements arranged non-collinearly, the transmitting andreceiving elements being disposed on a common substrate.

[0303] The invention further provides the use of an electromagneticantenna array in a method of obtaining positional information relatingto an object, the array including at least three receiving elementsarranged non-collinearly such that there is an axis about which thearray is asymnetrical.

[0304] The invention provides for a vehicle substantially as hereindescribed and with reference to the accompanying drawings.

[0305] Additionally, the invention provides a control system for airbags in a motor road vehicle substantially as herein described and withreference to the accompanying drawings.

[0306] The invention also provides for apparatus for a land vehiclesubstantially as herein described and with reference to the accompanyingdrawings.

[0307] The invention provides for a device for obtaining informationabout objects within or through a wall substantially as herein describedand with reference to the accompanying drawings.

[0308] The invention also provides for a hand-held tool substantially asherein described and with reference to the accompanying drawings.

[0309] There is also provided an electromagnetic microwave antenna arraysubstantially as herein described and with reference to the accompanyingdrawings.

[0310] There is also provided apparatus for generating an image ofobjects within or through a solid object substantially as hereindescribed and with reference to the accompanying drawings.

[0311] There is also provided apparatus, optionally for use on a (forexample, land) vehicle, for obtaining positional information relating toan object substantially as herein described with reference to theaccompanying drawings.

[0312] There is also provided a method for controlling deployment of airbags in a vehicle substantially as herein described.

[0313] Features of any aspect of the invention may be combined with orinterchanged with features of any other aspect as desired. Methodfeatures may be applied to apparatus aspects and vice versa. Featureswhich are provided independently may be provided dependently, and viceversa.

[0314] Although the embodiments of the invention that will be describedbelow operate by radiating ultra-high-frequency, microwave or millimetrewave radiation, in principle a range of other types of signals could beused in alternative embodiments. For example, embodiments could beconstructed which operate in other parts of the RF spectrum, they coulduse light, or they could use sound waves such as ultrasound.

[0315] Preferred features of an embodiment of the invention will now bedescribed in detail, purely by way of example, and with reference to theaccompanying drawings, in which:

[0316]FIG. 1 is a diagram showing the path of signals from atransmitting element to receiving elements of an embodiment of theinvention;

[0317]FIG. 2 is a representation of an antenna array of transmitting andreceiving elements being a component of an embodiment of the invention;

[0318]FIG. 3 is a representation of the main circuit elements of anembodiment of the invention;

[0319]FIG. 4 is a diagram illustrating the process of trilateration;

[0320]FIG. 5 is a flow diagram of the processing steps carried out byapparatus embodying the invention;

[0321]FIG. 6 shows an antenna array for use in an alternative embodimentof the invention;

[0322]FIG. 7 is a graph illustrating the outputs of the array of FIG. 6;

[0323]FIG. 8 shows an installation of apparatus embodying the inventionin a motor road vehicle;

[0324]FIGS. 9 and 10 are, respectively, side and plan views of thevehicle of FIG. 8 illustrating a warning zone of the apparatus;

[0325]FIG. 11 shows an installation of apparatus embodying the inventionfor controlling deployment of vehicle air bags; and

[0326]FIG. 12 shows an implementation of apparatus embodying theinvention implemented in an integrated unit.

[0327] Principles of Operation

[0328] With reference first to FIG. 1, apparatus embodying the inventionincludes a transmitting element 10 located on an axis 20. In thissimplified diagram, just two receiving elements 12, 14 are shown,located equidistantly on opposite sides of the axis 20. The transmittingelement 10 and the receiving elements 12, 14 are located on a commonplane disposed normally to the axis 20.

[0329] Radiation is emitted from the transmitting element 10 in a broadspread into a detection field of the apparatus. A portion of theradiation strikes and is reflected or scattered from first and secondobjects 16,18 in the detection field.

[0330] The first object 16, in this example, is located on the axis 20.A portion of the radiation emitted from the transmitting element 10travels along the axis 20 and strikes the first object 18. Some of thisradiation is reflected back to strike the receiving elements 12,14. Thetotal distance travelled by the radiation from the transmitting element10 to each of the receiving elements 12,14 is equal. As a consequence,the reflected radiation is received by the two receiving elements 12,14simultaneously. Provided that the speed of propagation of the radiationis known, and the round-trip time can be measured sufficientlyaccurately, the distance from the elements 10,12,14 to the first object14 can be determined.

[0331] The second object 16 is located away from the axis 20. As before,a portion of the radiation emitted by the transmitting element 10strikes the object 16, and some is reflected back to each of thereceiving elements 12,14. However, the length of the path followed bythe radiation is less in the case of radiation that strikes the firstreceiving element 12 than it is for radiation that strikes the secondreceiving element 14. This means that there is a delay between detectionevents in the first and second receiving elements 12,14. The length ofeach of the two reflected radiation paths can be determined directlyfrom the total round trip time for the radiation. Once the differencebetween the lengths of the paths is known, it is a straightforwardproblem in trigonometry to calculate the angular position of the secondobject 16.

[0332]FIG. 1 shows only two receiving elements 12,14 arranged in a linewith the transmitting element 10. This is sufficient to determine theposition of an object in two dimensions. The principle can be extendedto three dimensions through use of one or more additional receivingelements. FIG. 2 shows one possible arrangement of a transmittingelement and receiving elements, as is used in the embodiment that willnow be described.

[0333] Antenna Array

[0334] With reference to FIG. 2, there is shown a diagrammaticrepresentation of an antenna array 30 suitable for use in an embodimentof this invention.

[0335] In this embodiment, the radiation generated by the apparatus isconstituted by RF signals in the microwave band, and the antenna array30 is constructed accordingly.

[0336] The antenna array 30 is constructed on a substrate 32. In thiscase, the substrate is a block of plastic or glass-fibre compositematerial having a flat supporting surface. In order that embodiments ofthe invention are available for use where space is restricted, theantenna array is compact, having a peripheral size of approximately10×12 cm. Antenna elements are formed on the supporting surface of thesubstrate as conductors printed onto the surface. The antenna elementsmay be dipoles (for example, bow-tie dipoles), TEM horns, microstrippatches, stacked patches, or any other compact element or conductivestructure suitable for operating at the required signal frequency.

[0337] In this embodiment, the array 30 has five antenna elements intotal. Four of these elements are first, second, third and fourthreceiving elements 34,36,40,38 although other numbers of receivingelements, such as two, three, four or more, may be provided. The fifthelement is a transmitting element 42. The receiving elements 34,36,38,40are disposed at the vertices of a trapezium-shaped (which may, in aspecial case be rectangular) locus, and with more elements these couldbe disposed at the vertices say of a trapezoid or cuboid. Thetransmitting element 42 is disposed at the centre of the same locus.

[0338] For many applications, the size of the antenna array must be keptto a minimum. For example, the spacing between the elements may be inthe order of no more than a few centimetres, say between 1 and 10 cm,preferably between 3 and 8 cm. A hypothetical axis corresponding to theaxis 20 discussed with reference to FIG. 1 can be considered to extendnormal from the supporting surface through the centre of thetransmitting element 42. For reference below, the spacing between thefirst and second receiving element will be denoted D₁₂the spacingbetween the second and third receiving element as D₂₃, and so forth.

[0339] As a specific example, if the apparatus is designed for operationwith signals of frequency in the region of 6.5 GHz, the antenna elementsmay be dipoles of approximately 18 mm in length, and may be fed with abalanced line feed.

[0340] In an alternative form of construction, the antenna elements maybe located within a dielectric radome. Associated signal processingcircuitry may also be located within the radome in order to provide theapparatus as a self-contained package.

[0341] Turning now to FIG. 3, the circuit elements of the apparatusembodying the invention will now be described.

[0342] The apparatus includes a control and processing stage 66 thatcontrols the operation of other components of the apparatus. The controland processing stage 66 has a data output that sends data relating tothe position of one or more objects located within the detection fieldof the apparatus. Such data may be received by a terminal unit 90,possibly including an alarm, for further processing, for display to auser, and/or for transmission to a remote system, as required in anyparticular application.

[0343] A pulse generator and filter stage 46 is connected to thetransmitting element 42 of the array 30. The pulse generator may, forexample, be implemented using step-recovery diodes (“SRDs”), GaAs FETs,or SiGe transistors, the aim being to produce a sharp pulse waveform,which is then filtered on transmission to generate the transmittedsignal. Preferably, the rise and fall time of the waveform is in theorder of less than 0.5 ns. Each of the receiving elements 34,36,38,40 ofthe array 30 is connected to a respective filtering and amplifying stage48,50,52,54. The received signal is filtered to generate the outputsignal. Each of the filtering stages 46; 48,50,52,54 includes a bandpassfilter in the signal path from the transmitter to the transmittingelement 42 and from the receiving elements 34,36,38,40 to the receivercircuitry. Filtering is a standard technique used to ensure that thegenerated signal is suitable for the antennas, and for compliance withregulatory requirements.

[0344] Sampling Circuitry and Delay Lines

[0345] Signals from each of the filtering and amplifying stages48,50,52,54 are fed to a signal input of a respective switched samplingstage 58,60,62,64. The output of each switched sampling stage58,60,62,64 is connected to a respective input of the control andprocessing stage 66. Each switched sampling stage 58,60,62,64 has a gateinput, which, when activated by a suitable signal, passes signals on theinput line onto the output line. Each of the gate inputs is connectedthrough a respective delay line 68,70,72,74 to a common strobe line 76.The strobe line 76 is fed with signals from a sampling strobe signalgeneration stage 78. Each of the delay lines 68,70,72,74 imposes a delayas near as possible identical to each other on signals. The delay lines68,70,72,74 may be constructed as lumped capacitors and inductors, butmore preferably are equal lengths of printed transmission line of lengthL_(d). This delay will be referred to as t_(d), and will be discussedfurther below.

[0346] A timing signal generator 80 of the apparatus has two outputlines 82,84. A first of the output lines 82 is connected to a controlinput of the pulse generator and filter stage 46. A second of the outputlines 84 is connected to a control input of the sampling strobe signalgeneration stage 78. A control input of the timing signal generator 80receives signals from the control and processing stage 66. The timingsignal generator 80 operates to generate pulses at both the transmittertrigger instants and the receiver trigger instants.

[0347] In this embodiment, the timing signal generator 80 includes anoscillator and logic components. The oscillator includes acrystal-controlled clock, an output of which is fed to the logiccomponents. Upon initiation of a timing cycle, the logic components usethe signals received from the clock to generate a linear ramp signal.The linear ramp signal is fed to one input of a fast comparator, theother input of which is fed with an external voltage source. Thecomparator has an output upon which a signal is generated that isindicative of the relative magnitudes of the signals on its two inputs.Thus, the signal on the output changes its state after the initiation ofa ramp cycle at a time interval which is dependent upon the externalvoltage. The timing signal generator 80 stage is configured to generatea signal on its first output line upon initiation of the timing cycle,thereby creating a transmitter trigger instant, and on its second outputline upon the change of state of the comparator output thereby creatinga receiver trigger instant.

[0348] Alternatively, two comparators may be provided, each of which hasone input connected to a different external voltage source, and theother input connected to the ramp signal. The first comparator generatesan output signal when the ramp exceeds a first voltage, thereby creatinga transmitter trigger instant, and the second comparator generates anoutput signal when the ramp exceeds a second voltage, thereby creating areceiver trigger instant. In this way, even if the ramp signal drifts upor down, the interval between the transmitter trigger instant and thereceiver trigger instant will remain constant.

[0349] In an alternative embodiment, the timing signal generatorincludes first and second crystal-controlled clocks, the secondoperating at a frequency slightly below that of the first. Thus, thereis a slow variation in phase between the two clocks, whereby a slowlyvarying time delay can be generated.

[0350] Either of the two above-described embodiments (morestraightforwardly in the case of the former) permit and allow random orquasi-random variation in the timing of the transmitter and receivertrigger instants. This may be achieved, for example, by phase modulationof the timing signals, level shifting of the ramp, time modulation ofinitiation of the timing cycle or random signal inversion.

[0351] In alternative embodiments, the switch circuits may operateindependently or be triggered in common. During post-processing of thesignals, symmetrical leakage signals between the transmitting element 42and the receiving elements 34,36,38,40 can be used to correct for anydifferences between the timing of the various switched sampling stages58,60,62,64.

[0352] A most important consideration in the design of the timing andsampling stages is that inter-channel timing errors are minimised sothat an accurate comparison can be made of the times at which signalsare received by the various channels.

[0353] The filter stage 46 is designed to ensure that the signal fed tothe transmitting element 42 causes signals to be radiated that meetappropriate regulatory requirements, for example, in respect of theirpower and/or frequency, and to ensure that the signals are unlikely tointerfere with nearby equipment such as conmunication or sensingdevices. The filter stage 46 may be implemented using a known broadbandamplifier, associated with microstrip or lumped-element filters,selected to pass signals of frequency in the operating range of thedevice.

[0354] The switched sampling stages may suitably be implemented usingswitching diodes such as Schottly diodes. These may be configured in,for example, a bridge arrangement well-known to those skilled in thetechnical field, and are configured to be triggered by pulses generatedby the timing signal generator 80.

[0355] The control and processing stage 66 is constructed in accordancewith the specific requirements of the particular application in which anembodiment of the invention is to be used.

[0356] Sequence of Operation

[0357] A flow diagram of the processing steps carried out by apparatusembodying the invention is shown in FIG. 5. This illustrates the mannerin which signals from four receiving elements are filtered, digitised,and amplified. Then, pairs of signals are combined in a truncatedcross-correlation process, to provide a single output signal. Thissignal is then processed as required by any particular application. Suchprocessing might include target detection, measurement and tracking, andselection, before finally generating a report.

[0358] The sequence of operation of a preferred embodiment will now bedescribed.

[0359] When a scanning sequence is to be initiated, the control andprocessing stage 66 applies an enabling signal to the control input ofthe timing signal generator 80. The timing signal generator 80 thengenerates a train of pulses on both of its output lines 82,84. On thefirst output line, the pulses occur at times which will be designated ast₁, t₂, t₃, and so on. The pulses on the first output line 82 triggerthe pulse generator and filter stage 46, and consequently cause a signalto be emitted by the transmitting element 42. These times t₁, t₂, t₃, .. . will therefore be referred to as “transmitting trigger instants”.The transmitting trigger instants can be generated at regular intervals,but this is not a requirement. Some other predetermined pattern ofintervals may be used, or they may even be generated randomly. However,in this example, it should be assumed that the separation between thetransmitting trigger instants is an approximately constant time t. Thetime at which each of the transmitter trigger instants is generated isdetermined by programming of the control and processing stage 66.

[0360] On the second output line 84, the pulses trigger the samplingstrobe signal generation stage 78. The time of occurrence of pulses onthe second output line 84 will therefore be referred to as “receivingtrigger instants”. In the preferred embodiment, there is a correspondingreceiving trigger instant for each transmitting trigger instant,although in other embodiments there may be a plurality of receivingtrigger instants for each transmitting trigger instant, typically eachat a different delay.

[0361] The receiving trigger instants occur in a sequence which will berepresented as r₁, r₂, r₃, where r₁=t₁+T₀+T, r₂=t₂+T₀+2T, r₃=t₃+T₀+3Tand so forth. In the above, in this embodiment, T₀ is a constant whichmay be greater than or less than or equal to 0 and T is a non-zeroconstant.

[0362] In this embodiment, T is a constant greater than 0. However, Tmay be chosen to be less than or equal to 0, whereby at least onereceiving trigger instant occurs before or simultaneously with a firsttransmitting trigger instant. Signals received from the receivingelements 34,36,38,40 during this receiving trigger instant can beanalysed to obtain a sample of noise prior to transmission. Also,signals derived from a simultaneous receiving and transmitting triggerinstant can be analysed to obtain a sample of transmission leakagebetween the transmitting element 42 and the receiving elements34,36,38,40. The results of these analyses can be used to facilitateanalysis of signals received during subsequent operation of theapparatus.

[0363] In alternative embodiments, the values of T may be chosen tofollow a predetermined but discontinuous or partially continuoussequence.

[0364] In cases where T>0, as the sequence continues, each receivingtrigger instant occurs at an increasing time after the correspondingtransmitting trigger instant. That is to say, as n increases the valueof r_(n)-t_(n) also increases. The sequence continues until the value ofr_(n)-t_(n) reaches a predetermined maximum (typically in the order ofseveral or several tens of nanoseconds). The maximum time corresponds tothe longest expected round-trip time for a signal emitted by thetransmitting element 42 to be reflected from an object in the field ofdetection of the apparatus, and received by the receiving elements34,36,38,40, a time governed by the maximum range of operation of theapparatus.

[0365] In an alternative embodiment, the value of T₀ is comparativelylarge and positive, and the value of T is negative. In such embodiments,the value of r_(n)-t_(n) starts at a maximum when n=0, and decreases asn increases.

[0366] At a fixed time interval after each transmitting trigger instant,the pulse generator and filter stage 46 generates a transmitting signal.The signal is filtered to meet the appropriate regulatory standards, andis then passed to the transmitting element 42 from which it is radiatedinto the field of detection of the apparatus. In this embodiment, theemitted signal may have a frequency of 2.45 GHz or 6.5 GHz, which ismade available by European and US regulatory authorities forapplications such as this.

[0367] At a fixed time after each receiving trigger instant, thesampling strobe signal generation stage 78 generates a pulse. This pulseis passed to the gate input of each of the switched sampling stages58,60,62,64 through the respective delay line 68,70,72,74. The effect ofthis pulse arriving at the gate inputs is to close simultaneously eachof the switch sampling stages 58,60,62,64 for an aperture time ta,during which the signals on the input of each is passed to its output.During the aperture time ta, the signals received from receivingelements 34,36,38,40 and processed by the filtering and amplifyingstages 48,50,52,54 are conveyed to the control and processing stage 66.

[0368] Avoidance of Crosstalk Between Samplers

[0369] It is likely that there will be some signal leakage from theinput of the switched sampling stages 58,60,62,64 to their gate inputs.The purpose of the delay lines 68,70,72,74 is to avoid this resulting incrosstalk between signals from the various receiving elements. It willbe seen that the minimum time taken for a signal to pass from oneswitched sampling stage 58,60,62,64 to another is not less than 2td(since the signal must pass through two of the delay lines). The valueof td is therefore chosen such that ta<2td (and more preferably ta<td)whereby no signal can propagate from one switched sampling stage toanother within the aperture time ta. This arrangement permits theswitched sampling stages 58,60,62,64 to be triggered from a commonsignal. This is in contrast to a more conventional arrangement in whicha separate strobe pulse generator is used to generate a signal for eachindividual switched sampling stage; a more complex and costlyarrangement that can lead to uncertainty in the relative timing betweentriggering of the various switched sampling stages 58,60,62,64.

[0370] In embodiments in which the delay lines are constituted asprinted transmission lines, the value of Ld (as defined above) is givenby the formula

Ld>VI.ta/2

[0371] where VI is the speed of propagation of signals in thetransmission line. For example, if the value of ta is in the order of 50ps, the value of Ld may be in the region of 10 mm.

[0372] Calculation of Object Ranges

[0373] As will be understood, when a signal is conveyed to the controland processing stage at a receiving trigger instant n, it is known thatthe signal had a round-trip time from the transmitting element 42 to thereceiving element in a time of T₀+nT+C. In this expression, C is aconstant that represents relative total delay between the transmittingtrigger instants and the start of the transmission, and the receivingtrigger instants and the start of the receiving aperture time. Thedistance of the object from which the signal is reflected is thereforegiven by c(T₀+nT+C)/2. As will become apparent in due course, it isimportant to note that the distance of the object can be calculated by aformula that does not depend on t, nor does it depend upon the absolutevalue of t_(n).

[0374] As a particular example, for a device operating at or around 6GHz, T may be approximately 160 ps and ta may be approximately 50-80 ps.

[0375] Calculation of Angle to Object—Azimuth and Elevation

[0376] As was discussed with reference to FIG. 1, a proportion of theenergy emitted by the transmitting element 42 at each transmittingtrigger instant is reflected back towards the apparatus and is receivedby the receiving elements 34,36,38,40. In this embodiment, the receivingelements 34,36,38,40 are arranged in a two-dimensional array.Consequently, the reflected energy arrives at the receiving elements34,36,38,40 at different times dependent upon the object'sthree-dimensional location with respect to the apparatus.

[0377] Assume now that there is an object located at a distance R5 fromthe transmitting element and at distances R1, R2, R3 and R4 from thereceiving elements 34,36,40 and 38 respectively.

[0378] If an object is located on the axis of the array 30 (sometimesreferred to as “the boresight”), it is equidistant from all fourreceiving elements 34,36,38,40, with the result that it will arrive atall four receiving elements 34,36,38,40 simultaneously.

[0379] On the other hand, if the object is located off-axis, for exampleat a location with elevation azimuth φ and elevation θ, there will be atime difference of approximately Dv sin θ/c between the signals arrivingat vertically spaced receiving elements of the array 30 and a timedifference of approximately Dh sin φ/c between the signals arriving athorizontally spaced receiving elements of the array 30 where Dv and Dhare, respectively, the vertical and horizontal distance betweenreceiving elements in the array 30. In the above, c is the speed oflight.

[0380] More specifically, the times at which the reflected signals willbe received by the receiving elements are times Tr1, Tr2, Tr3, Tr4,respectively:

[0381] Tr1=(2 R5+(−D14 sin (θ)+D12 sin (φ))/2)/c

[0382] Tr2=(2 R5+(−D23 sin (θ)−D12 sin (φ))/2)/c

[0383] Tr3=(2 R5+(D23 sin (θ)−D34 sin (φ))/2)/c

[0384] Tr4=(2 R5+(D14 sin (θ)+D34 sin (φ))/2)/c

[0385] where R5 is the distance between the transmitting element 42 ofthe array 30 and the object from which the received signals have beenreflected, these equations being approximations for small angles.

[0386] The above equations can be solved to obtain the angles θ and φ.

[0387] It will be observed that there is some redundancy in theinformation received from an array 30 of four or more elements. This maybe dealt with by selection or by an averaging or a weighting process.

[0388] A Cartesian coordinate (X, Y, Z) can then be calculated. If R isthe range to the object (calculated by multiplying the total round triptime for a reflected signal by the speed of light) then:

[0389] X=R cos θ sin φ

[0390] Y=R cos θ cos φ

[0391] Z=R sin θ

[0392] In order to determine the arrival time of the reflected signals,the control and processing stage analyses signals appearing from theswitched sampling stages 58,60,62,64, as will now be discussed.

[0393] In general, the distance D is small in comparison with thedistance between the array 30 and the object from which the signals havebeen reflected. Thus, the assumption is made that the reflected signalcomprises principally plane waves, and that the signal does not changesignificantly between one receiving element and another. It is alsoassumed that each of the receiving elements 34,36,38,40 will reactsubstantially identically to the signal. Moreover, each receivingelement has a small angular resolution with respect to the object.

[0394] Time Scale Stretching

[0395] Referring again to FIG. 3, each of the switched sampling stages58, 60, 62, 64 is activated at a time r_(n)=t_(n)+T₀+nT, for n=1, 2, . .. m. When a sampling stage is activated, it samples the input waveformand holds that value until the next receiving trigger instant r_(n+1).As described above, the value of r_(n)-t_(n) is arranged to increase (ordecrease) as a function of n. It therefore takes t/T repetitions tocomplete the waveform, where t is the mean separation between thetransmitter trigger instants.

[0396] Assuming that the reflected signal is essentially unchangedbetween trigger instants (such as is the case if the object does notmove significantly relative to the transmitting and receiving elements),then a strobe effect takes place which results in the detected signalbeing stretched by a factor of t/T. In this way the output of thesampling stages is of duration greater than that of the received signalsby a factor t/T and of frequency less than that of the received signalsby a factor t/T.

[0397] The last-described arrangement stretches the received signal intime without changing the signal shape. This is beneficial because itreduces by a factor of t/T the frequency at which the processing stage66 operates. The value of t/T may be considered as a constant divisor offrequency and multiple of time.

[0398] In the particular case where t=T, the output of the switchedsampling stages 58, 60, 62, 64 would be at the same frequency as theinput signal. However, if t≠T, then the output of the switched samplingstages 58, 60, 62, 64 would be at a frequency that is reduced by afactor of t/T. For example, if t=100 ns and T=1 ps then the factor t/Twill be 10 ₅. For an incoming signal frequency of 5 GHz at the receivingelements, the output of the switched sampling stages 58, 60, 62, 64 willbe a signal of 50 kHz.

[0399] Under the transformation described above, the shape of thereceived signals is preserved in the time domain, but their frequency isreduced by a common divisor. The resulting signals may be of afrequency, duration and data rate that can readily be processed bycomparatively low-cost hardware.

[0400] Frequency stretched signals from the switched sampling stages 58,60, 62, 64 are converted to digital form before being passed to thecontrol and processing stage 66. The particular processing then carriedout on the signals, and the output data generated by the control andprocessing stage 66, is highly dependent upon the particular applicationand intended function of any given embodiment of the invention.

[0401] Determining a Time Interval Using Truncated Cross-Correlation

[0402] In this embodiment, the analysis is performed by a modificationof a known method of cross-correlation. The analysis is carried out uponsignals which are assumed to contain a plurality of cycles with similar,but non-identical, envelope shapes. Conventional cross-correlationmethods can give ambiguous results when carried out on signals of thistype.

[0403] Following on from the assumption described in the last-precedingparagraph, it is possible to determine the time separation of thereceiving instants in a process that compares the signals derived fromtwo of the receiving elements 34,36,38,40, and determines the timedifference at which these signals appear most similar.

[0404] Conventional cross-correlation between two signals includes thestep of summing the products of the two signals over a period (theinterval of correlation) corresponding to the duration of the waveformcontained in the signals. This step is carried out for a series of casesbetween which one of the signals is shifted in time with respect to theother over a range which, in conventional cross-correlation, is alsocomparable with the duration of the signal.

[0405] For signals that include a plurality of cycles of similar shape,the results produced by conventional cross-correlation can be ambiguous.The method used in the present embodiment (which method will be referredto herein as “truncated cross-correlation”) takes advantage of the factthat the maximum time difference between the same reflected signal beingreceived by any two of the receiving elements is the time taken for thesignal to propagate the distance between adjacent receiving elements.This can be expressed as a value D/c. In the present embodiment, therange over which one signal is shifted with respect to the other islimited to this value. This results in a signal containing only one or afew peaks, the actual number being determined by the ratio of theelement separation (D) to the wavelength (λ) of the transmitted signal.Thus, the method of truncated cross-correlation is optimised by takinginto account the geometry of the apparatus, rather than by reference tothe signal wavelength.

[0406] More specifically, the truncated correlation process is asfollows:

[0407] Assume that the received signal from element 1 is an amplitudemodulated wave V₁=V₀.P(t)₁ and that the received signal from an element2 is an amplitude modulated wave V₂=V₀.P(t)2. P(t) is substantiallynon-zero for a time period −t_(p) to t_(p) for both waveforms.

[0408] Internally within a data processing system, each of the waveformsis represented by a series of numbers X(n) and Y(n) that represent,respectively, the values of V₁ and V₂ at time t(n) where t(n)=t₀+ndt anddt=Aλ/kc, and where

[0409] A is a constant which depends on the amount by which the signalhas been stretched (typically A=t/T)

[0410] c is the speed of light, and

[0411] k is an integer between 2 and (for example) 20 representing thenumber of digital samples per wavelength.

[0412] The truncated correlation function is:${C(m)} = {\sum\limits_{nmin}^{nmax}\left( {{X(n)} \cdot {Y\left( {n + m} \right)}} \right)}$

[0413] for values of m from −KD/λ to kD/λ, where:

[0414] m is the shifting index, which is an integer between −kD/λ andkD/λ, representing the index of truncated correlation, and

[0415] nmax and nmin define the interval of the correlation, typically afew cycles (say, 2 cycles) of the waveform.

[0416] Thus it will be appreciated that in the truncatedcross-correlation according to the present embodiment, the range overwhich one signal is shifted with respect to another is truncated, incomparison to conventional cross-correlation. The range of the shift isrelated to the separation of the receiving elements. For example, atransmitted pulse may have a length of 2λ whereas the separation betweenreceiving elements may be λ/2. Since it is known that the maximum delaybetween the two signals is equal to the time it would take thetransmitted signal to travel between the two receiving elements, theindex of correlation can be limited to a value corresponding to thatmaximum delay. Thus, the range of shift is not related to the durationof the signals themselves, but rather to the distance between receivingelements.

[0417] It will be noted that the interval of correlation is alsotruncated, in this case being bound by the values nmin and nmax, thesevalues being selected to limit the cross-correlation to the useful partof the waveform.

[0418] As will be understood, this correlation takes place in a shortspace of time in order that the system as a whole can generate imagesand other data sufficiently quickly. To achieve this, a correlatingfunction such as this can be carried out directly in hardware, forexample, by digital signal processors connected to the switched samplingstages through analogue to digital converters. Alternatively, a softwareprogram running on suitable hardware, such as a microprocessor, maycarry out the correlating function.

[0419] This truncated cross-correlation of the signals derived from anytwo of the receiving elements 34,36,38,40 produces a maximum at a valuecorresponding to the sine of the angle between the axis 20 and a line tothe location of the object projected onto the plane in which thereceiving elements lie. By performing such correlation with various ofthe receiving elements in combination, the angular position of theobject in three dimensions can be obtained. In the preferred embodiment,the receiving elements have wide beams and little angular resolution,and angular positions are determined by trilateration, that is, byprecise path length comparison, as illustrated in FIG. 4, in which pathdistance D sin θ is shown as a function of offset angle θ and thedistance D between the receiving elements R1, R2.

[0420] In the light of the above, it will be understood that preferredembodiments may be considered as a “delay monopulse” radar (with delaybeing measured directly in the correlation procedure).

[0421] Enhancing Resolution of the Array

[0422] The angular resolution of the array 30 is relatively poor withrespect to objects lying close to the plane of the array. Furtherprocessing steps may be carried out to improve resolution in thisrespect. For example, the relative amplitudes of signals received byeach of the receiving elements 34,36,38,40 can be compared.

[0423] In cases where the array is moving with respect to objects in itsfield of detection, the time at which reflected pulses are received bythe receiving elements will vary from one scan sequence to the next. Thechanges can be analysed to gain information about the position andmotion of the objects. One effect of this is to enhance the ability ofthe array to resolve closely spaced objects.

[0424] As will be understood, the correlation sequence described abovewill produce a maximum at a delay that corresponds to the intervalbetween the reflected signal being received at the two receivingelements concerned. This delay can be used to determine an angulardirection of the object from which the signal was reflected. Acombination of the results of the correlation being applied to two ormore non-collinear pairs of receiving elements can be used to derive theposition of the object in three dimensions using the formulae set forthabove.

[0425] It is possible to carry out a first, relatively coarsecross-correlation of the signals in order to obtain an approximate valuefor the angle at which the object is located, and then to carry out afurther, relatively finer correlation, using a larger number of samples,around the angle first estimated, and thereby refine the estimate.

[0426] Ameliorating the Effect of Grating Lobes

[0427] If the separation between receiving elements D is greater thanλ/2 then grating lobes may occur, which may in turn give rise to falsemaxima. These false maxima can be distinguished by generating asignificantly reduced correlation between the channel pairs, therebyreducing the difficulties normally associated with narrow-band arrays.Grating lobes may also be reduced or eliminated by reducing the spacingbetween the elements to less than one half of the dominant wavelength ofthe signal (that is, by setting D<λ/2); however, the cost of this isreduced angular resolution and increased inter-element coupling.

[0428] In preferred embodiments, the occurrence of grating lobes iscontrolled by using different horizontal separations for two pairs ofreceiving elements 634 . . . 640 (that is to say, D₁₄≠D₃₄). An array 630embodying this modification is shown in FIG. 6. In such an array, theposition of the principal correlation maximum will be the same for eachpair, but the position of the false maxima caused by the grating lobeswill differ to an extent dependent upon the difference in spacingbetween the element pairs.

[0429] As a specific example, if D₃₄=λ and D₁₂=3λ/4, the correlationpeaks arising from the grating lobes of elements 1 and 2 coincide inangle with the correlation zeros arising from the grating lobes ofelements 3 and 4. If the truncated correlation functions of the twopairs of elements are multiplied together, the “true” peaks that arisefrom the correlation are enhanced, while the peaks that arise from thegrating lobes will be greatly reduced in amplitude. A graph illustratingthe output of both pairs of elements individually and as combined asdescribed above is shown in FIG. 7. In FIG. 7, trace A shows the angularresponses of a pair of receiving elements, while trace B shows thecombined response illustrating the result of carrying out themultiplication described above.

[0430] As has been discussed above, the value of D may be in the orderof 10⁻² m, whereby the value of D/c is in the order of 10⁻¹⁰ s,mandating a signal frequency in the range of several GHz to achievesatisfactory resolution. While it is possible to carry out signalprocessing upon signals having a frequency and duration of this order,the apparatus required is considerably more costly than may be the casefor apparatus having lower speed capabilities. In many embodiments, costis of considerable importance. For this reason, a processing step oftime scale stretching is used to reduce the frequency requirements ofthe processing circuitry.

[0431] The transmitted signal is of duration such that its length inspace is of magnitude similar to that of the objects that it isprimarily intended to detect, and also of the same magnitude as thespacing between the receiving elements 34,36,38,40. A consequence ofthis is a small likelihood of combined reflections from several objectsadding to produce a combined reflected signal. This allows severalobjects to be resolved within the detection region of the apparatus,provided that these are not located very close together.

[0432] For objects that are close to the plane in which the elements arelocated, the sensitivity of the relative timing of the received signalsto small changes in angular position of the object will be relativelypoor. In this situation the sensitivities of each receiving element canbe made different, allowing further angular discrimination on the basisof the received signal amplitude.

[0433] Thresholding and Pattern Matching

[0434] In a first example of an enhancement to a system as describedabove, the control and processing stage is operable to determine theamplitude of the reflection from each object in addition to itsposition. It may then be operable to determine whether the object is ina position and of a cross-section sufficient (that is in excess of athreshold) to warrant say activating an alarm. In the example of awarning system for a vehicle driver, an alarm may be activated in suchcircumstances to warn a driver of the proximity of a hazard.

[0435] Since the angular position of an object determines thesensitivity of the receiving elements in the direction of that object,determination of the amplitude of reflection typically is made after theangular position of the object has been determined in thecross-correlation procedure described above. Suitable further processingof the received signals may proceed in a process as will now bedescribed. First, the amplitude of the maximum value obtained from thecorrelation (that is, the value used to calculate the angular positionof a detected object) is obtained. The gain of the array, as applicableto signals received from that angular position is then determined,typically from a look-up table stored in memory of the control andprocessing stage 66, and the amplitude value is divided by the gain toproduce an angle-independent amplitude value. The angle-independentamplitude value is then multiplied by a value proportional to the fourthpower of the calculated distance of the object (i.e. by a valueproportional to D₅₄). This resulting value is proportional to the radarcross-section of the object, and will be referred to as the “objectamplitude”.

[0436] In order to decide whether the object detected in thecross-correlation process is sufficiently significant to warrant (say)activating an alarm, in typical embodiments the object amplitude iscompared with a threshold, and the activation (or other action) isinitiated in the event that the object amplitude exceeds the threshold.In such embodiments, the threshold may be determined by varioustechniques. In the simplest case, it may be a fixed value.Alternatively, it may be varied in accordance with a variety of rules.As a first example of such a rule, the threshold may be varied as afunction of the round-trip time of the signals (effectively, as afunction of the range R₅). Alternative or additional variation to thethreshold may be made as a function of, amongst other possibilities,levels of noise in the received signals, (in appropriate embodiments)the parameters of operation of a vehicle, or an externally appliedsignal, for example a signal applied by a user control.

[0437] Alternatively or additionally, the signals received may besubjected to pattern matching analysis. For example, they may becompared qualitatively to pre-determined signal patterns derived fromsignals reflected from known classes of object. Such analysis mayinclude comparison of the shape of the received signals with a pluralityof prototype signals previously measured and stored in memory accessibleby the processing stage, or may include identifying characteristicfeatures of the received signals such as their duration, their amplituderise and fall times and their frequency spread. In each case, the matchfound indicates that the signal has been reflected from a particularclass target objects. Then a match is found, or characteristic featuresare identified in the received signals, the processing stage may alterthe severity of a warning signal, or otherwise modify its output, inresponse to the class of objects identified. For example, inapplications for use on a vehicle, particular priority may be given tosignals that are identified as having been reflected from a person oranimal in the path of the vehicle.

[0438] Warning Zone

[0439] The processing steps described above can determine the3-dimensional position of an object within a detection field of thesystem. However, further, the detection region can be sub-divided into afirst zone in which detection events are considered to be significant,and a second zone in which they are not significant. Effectively, thefirst zone defines a warning zone of the system. The sub-division can becarried out by the control and processing stage 66, typically by asoftware program.

[0440] The control and processing stage 66 may operate to execute analgorithm that defines a 3-dimensional volume of space within thedetection field near to the array 30 as a warning zone. For example, thewarning zone may be defined to lie between spaced planes by specifyingthat it is bounded by minimum and maximum values of X, Y, and Zordinates in a Cartesian coordinate system within the detection field ofthe array. Alternatively, the warning zone may have an arbitrary shape,defined by a look-up table or a mathematical formula. Thus, the warningzone can have substantially any shape that can be definedalgorithmically, and can have any volume, provided that it is entirelycontained within the detection field.

[0441] The control and processing stage 66 is operative to issue awarning, for example at least one of an audible, a visual or a tactilewarning to a user upon detection of an object in the warning zone.

[0442] As a development of this embodiment, the control and processingstage 66 defines a plurality of warning zones. The warning zones may benon-coextensive (overlapping, separated or spatially different) and/oralternatively defined, by which it is meant that differentcharacteristics are used for determining whether an object is in therelevant warning zones. For example, different zones may be provided fordetecting different speeds or different sizes of objects. This can, forexample, be used to provide warnings of multiple levels of severity,depending upon the position or other characteristics of a detectedobject.

[0443] In another development of this embodiment, the control andprocessing stage 66 is operative to analyse characteristics of objectsoutside of the warning zone. Such characteristics may be, for example,size of the object, distance of the object from the apparatus and/or thewarning zone, direction of movement of the object relative to theapparatus and/or the warning zone, and relative speed of the object. Asan example, the control and processing stage 66 may be operative totrack objects outside the warning zone and to predict their entry intothe warning zone. The apparatus may be operative to issue a pre-warningbased on the analysis.

[0444] For example, if the apparatus is mounted on a vehicle (asdescribed below) in order to provide the driver with a parking aid, theapparatus may issue a pre-warning in the way described above if a largeobject is converging with the vehicle, even though that object may beoutside of the warning zone. This may be particularly desirable, forexample, if the object itself is heading for the vehicle in the samedirection that the vehicle is heading for the object, which may give anincreased risk of collision.

[0445] An Application to Land Vehicles

[0446] Apparatus embodying the invention can be installed in a motorroad vehicle 200, as shown diagrammatically in FIG. 8. In this example,the apparatus is intended to warn a driver of objects or obstructionsexternal of the vehicle. The apparatus includes an array and processingcircuitry mounted, as shown at 202 in a single enclosure within anon-metallic front bumper 204 of the vehicle 200. The array andprocessing circuitry may be as described below with reference to FIG.12.

[0447] The apparatus implements a shaped warning zone, as illustrated inFIGS. 9 and 10. In this example, the warning zone might be shaped as acuboid, as shown at 220, contained within a detection region 222 of theapparatus. A lower surface of the cuboid is spaced a short distance fromthe ground 228 on which the vehicle 200 is standing so that a warning isnot generated by the presence of very low kerbs or bumps in the road. Anupper surface of the cuboid is disposed at the height of the highestpart of the vehicle (for example, a radio antenna 208), plus someadditional height as margin for error. Similarly, the width of thewarning zone is the width of the vehicle plus a margin for error.

[0448] A Cartesian coordinate of an object within the detection fieldcan be expressed with, for example, the X-axis across the vehicle, theY-axis as fore and aft distance, and the Z-axis as height. Thus acuboidal warning zone can be defined as being all points that meet therequirement that:

(Xmin<X<Xmax; Ymin<Y<Ymax; Zmin<Z<Zmax)

[0449] where Xmin, Xmax, Ymin, Ymax, Zmin, and Zmax are eitherconstants, or are varied in response to changing operating conditions.

[0450] The apparatus may change the shape or size of the warning zone inresponse to changing vehicle operating conditions. The apparatusreceives and processes signals from sensors mounted on the vehicle thatdetect, for example, the vehicle's speed, steering and throttle input,ambient conditions, and so forth. For example, at low speed, distantobjects are of less concern, so the warning zone might be shortened, asshown at 224. Objects detected within the volume thus removed from thewarning zone might be ignored. Alternatively, that space might betreated as a second warning zone. The apparatus may generate a low-levelwarning when an object is detected within the second warning zone and ahigher-level warning for a closer object.

[0451] The warning zone could be defined to taper or to curve such thatit is wider further from the vehicle to take into account possiblevariations in the vehicle's course. For example, as shown in FIG. 10,the warning zone might be extended in a direction in which the vehicleis steering. As illustrated, an additional region 226 has beenintroduced into the warning zone of a right-turning vehicle.

[0452] As another example of apparatus, with reference to FIG. 11apparatus 1110 may be provided to monitor the internal volume 1100 of avehicle for the purpose (for example) of intruder monitoring or controlof air bags.

[0453] When such an embodiment is used as an intruder monitor, thewarning zone will typically be limited to a volume that is approximatelyco-extensive with the interior space 1100 of a vehicle. However, (withreference to FIG. 11) when used to control air bags, the warning zonewill typically include a plurality of regions 1112, each correspondingto a region that might be occupied by a passenger in a seat 1114 that isprotected by an air bag 1116. Upon detection of an accident, and therequirement to deploy air bags to protect the occupants of the vehicle,the apparatus determines which regions 1112 of its warning zone areoccupied, and deploy only the corresponding air bags. Additional regions1120 of the warning zone may also be defined close to the air bags 1116.In the event that the system detects that any such additional region1120 is occupied, deployment of the respective air bag will besuppressed to reduce the risk of injury to a person occupying thatspace.

[0454]FIG. 12 shows one arrangement in which apparatus including anantenna array and associated processing circuitry can be arranged.

[0455] The apparatus is constructed around a metal substrate 1210,formed as a rectangular plate. On one side of the plate is mounted anantenna 1212 array comprising several polymerplates, between which isdisposed a plurality of transmitting and/or receiving elements 1216. Onthe other side of the substrate 1210 is carried a printed circuit board1214 upon which is constructed electronic circuits that exchange signalswith the elements 1216. In this embodiment, the dimension a isapproximately 12 cm and the dimension b is approximately 10 cm. Theapparatus is therefore self-contained and compact.

[0456] In summary, preferred embodiments measure the position of objectsand determine directly whether an intruder is present, or whether ahazard exists due to the position of the object with respect to, forexample, an airbag.

[0457] Preferred embodiments consist in a fixed-aperture radar sensorfor vehicles, with a wide field of view which transmits short pulses,and use a multi-element antenna, a multi-channel swept-delay sampling ormixing receiver, a cross-channel delay processor, a digitally-stored orconstructed definition of an arbitrary 3-dimensional warning zone, and adigitally-stored or constructed description of the beam pattern of theantenna. Such a sensor can be used to measure the 3-dimensional positionof one or more objects within its field of view, determine whether eachlies within the warning zone, and determine the radar cross-section ofeach object.

[0458] The microwave position measuring sensor uses a small array ofantennas and a wideband signal to measure the positions or shapes ofobjects within its field of view by a radar-like technique. The signalis emitted by one element of the array, is scattered from objects in thefield of view, and a scattered portion is received a time interval laterat each of the receiving elements, the interval being proportional tothe distance to the object. The wide band signal (in one embodiment ashort pulse) allows multiple objects to be resolved in terms of distancefrom the sensor. Each object gives rise to one such signal received ateach element of the array, and the direction to that object iscalculated by measuring and comparing the time at which the relevantsignal is received at each antenna.

[0459] The advantage of this approach is that the need for precision indetermining the presence of an obstacle in the warning zone is met bymeasuring its angular position within a relatively broad antenna beam,rather than by using a larger antenna or higher frequency to provide aprecisely-tailored or narrow beam. This permits the use of a smallerfrequency/aperture ratio than hitherto.

[0460] The frequency can be chosen on the basis of cost of devices (lowfrequency is good), size of antenna (high frequency is good), receiverperformance (low frequency is good), scattering cross-section (dependson geometry, but wide relative bandwidth (high B, low F) reduces glint),and regulations.

[0461] The beam pattern of each array element is known and itsdescription is stored within the sensor's processing electronics. Adescription of a 3-dimensional warning zone is also stored to determinewhether an object is in a position where a warning is appropriate.

[0462] The field of view of the sensor, as determined by the antennabeam patterns, the transmitter power, the desired minimum detectableobject cross-section, and the noise level, contains the warning zone andexceeds it in maximum range in most directions.

[0463] Targets may be isolated by range resolution of centimetres, andtheir directions found by comparing reception delays between elements ofa small or minimum array. This yields a small device, adequate for anyshort-range applications, capable of 3D location of many range-separatedstatic targets and of extension to synthetic or inverse syntheticaperture processing for moving targets or a moving platform.

[0464] A single fixed wideband transmitter and three, four or more fixedreceiving elements form such an array. As an illustrative embodiment,the receiving array elements may be arranged at the corners of arectangle or cuboid, with the sides of the array similar to or less inlength than the wavelength, and the transmitter may be positioned nearthe centre of the array. The elements may be dipoles or microstrippatches, or other small radiating conductive structures, For ease ofmanufacture and wide bandwidth, stacked patches may be used withadvantage. The array may be constructed as a single assembly ofconductors printed on a glass-fibre or plastic substrate, or assembledwithin a dielectric radome with its associated electronics andprocessing computer.

[0465] In operation, these circuits are used to process the receivedsignals, and differ from conventional antenna array processing circuitsby replacing frequency shifting, or phase shifting at radio frequency,by frequency scaling. For each receiving element the output signalcontains frequency components each of which is derived from its originalradio frequency value by a constant divisor. The value of the divisordepends either on the rate of change of delay at which samples areobtained, or on the ratio of the target's approach speed to that oflight, or both.

[0466] An array of fixed wideband, wide beam antenna elements isprovided, suitable for fitting within assemblies such as car bumpers orportable enclosures, without radio frequency phase shifting or combiningcircuits, nor with frequency-shifting circuits or a carrier frequencylocal oscillator, but with a short pulse generator associated with atransmitting element and wideband, switched sampling circuits forfrequency scaling associated with each receiving element.

[0467] The switched sampling receiver circuits are designed with lowinter-channel timing errors to allow accurate relative timing of thereceived signals to be measured.

[0468] The imaging sensor also includes processing means which areoperable to (a) calculate timing of, and intervals between similarsignals received at each channel from objects, (b) calculate the 3Dposition of each object observed from such timing and offsets, (c)compare such 3D position with a stored or constructed 3D warning zone,(d) activate a warning or indication if such 3D position is inside suchwarning zone, (e) calculate the antenna gains in the direction of such3D position, and (f) calculate the radar cross-section of such objectfrom the amplitude of the signal received from it.

[0469] In addition to allowing remote detection and position measurementof obstacles, the operation of such a sensor at microwave frequencies orbelow allows a useful degree of penetration of solid materials. Thus thesensor may be placed behind the bumper of a car without requiring holesor special materials or treatments. It may also be positioned to detectand measure objects and obstacles behind other materials such as wood,plastic, concrete, brick and other nonmetallic materials. Metal or otherobjects may be detected within such nonmetallic materials. Measuredobjects may be stationary. Additional processing including targetclassification, tracking and imaging may be applied to improve thedetection and measurement of moving objects.

[0470] The application of such a sensor may include automotive obstacledetection outside a vehicle; providing a driver with a completesituation assessment near the car; occupant position sensing within avehicle; occupant identification and behaviour monitoring; collisionwarning for aircraft; landing aids for helicopters; fluid levelmeasurement; security sensors for buildings; surveillance sensors forsecure rooms and areas; manoeuvring aids for vehicles; earth movingequipment; traffic presence and movement; vehicle identification, etc.

[0471] The present invention has further application to embodiments thathave diverse applications. Several of these will now be described.

[0472] Since RF electromagnetic signals can penetrate buildingmaterials, embodiments of the invention that use such signals can beused to form positional images of objects behind walls and obstructionsof brick, stone, concrete, cinder block, wood, plasterboard etc.

[0473] Such objects may be located in a space behind such a wall orobstruction, allowing the user to detect and measure the position orheight of objects, people, animals, vehicles or surfaces in the space.This may have applications in security activities, search and rescueactivities (e.g. for earthquake, landslide or avalanche victims) andmany other activities in a built environment.

[0474] Such objects may also be located within the wall or obstructionitself, including reinforcing bar, studding, pipes, drains, beams andgirders, and voids, cracks, gaps, ducts etc. This is useful forinspection purposes, to detect the presence and map the positions ofsuch objects or voids etc., to facilitate works or repairs, to determinethe state of the wall or obstruction, to identify the nature of suchobjects or voids and so forth.

[0475] Embodiments may be used to discriminate between objectsexhibiting different radar cross sections, for example, pipes ofdifferent diameters or different materials, flat surfaces of differentmaterials, etc.

[0476] Further embodiments may be used as a prosthetic for those withimpaired eyesight, providing an obstacle sensor and directional warningdevice to warn of the approach of people, animals, obstacles, kerbstones(especially given the 3-dimensional nature of positional informationprovided), trees, walls, and so forth. In a similar manner, embodimentsof the invention may be used as a night vision aid, allowing thedetection and position measurement of objects in the absence of light,in fog or smoke.

[0477] Likewise, further embodiments may be used in addition to or inplace of optical or infra-red sensors in conjunction with unmannedvehicles such as robots in security applications including bombdisposal, in manufacturing, mining, warehousing and storage, tunnelling,decommissioning or construction of nuclear and other hazardous plant,demolition, building construction work, among other possibilities.

[0478] Additionally, the sensor may be used within a road vehicle todetermine the position of objects which may be the head or chest of anoccupant of the vehicle in relation to hazards associated with crashesand impacts, and with counter-measures such as airbags or belts. In thisconnection it may not be necessary to use the sensor itself with patternmatching techniques to classify the object, but merely to measure itsposition and motion. In this case the sensor is distinguished from priorart in that, by comparison with existing radar sensors, which measurethe distance between occupant and the airbag (or other item, as the casemay be) directly, and which are vulnerable to interference with the beamfrom objects carried, worn or inadvertently placed by the occupant, suchas arms, legs, feet, hands, books, newspapers, boxes, etc. Such anembodiment can measure the angular position of the object as well as thedistance and can be located in a position from which its beam is lesslikely to be interrupted.

[0479] It will be understood that the present invention has beendescribed above purely by way of example, and modifications of detailcan be made within the scope of the invention.

[0480] The Applicant asserts design right and/or copyright in theaccompanying drawings.

[0481] Each feature disclosed in the description, and (whereappropriate) the claims and drawings may be provided independently or inany appropriate combination.

[0482] Reference numerals appearing in the claims are by way ofillustration only and shall have no limiting effect on the scope of theclaims.

1. Apparatus for obtaining positional information relating to an object,the apparatus comprising: an array comprising a transmitting element anda plurality of receiving elements; a signal generating stage forapplying a series of pulses to the transmitting element to cause it totransmit a signal, such that at least a portion of the signal can bereflected from the object to be received by the receiving elements; adetection stage for detecting signals reflected to the receivingelements and for generating output signals representative of thereceived signals; and a processing stage operable by application of atruncated cross-correlation function to detect the interval betweensignals received by a plurality of the receiving elements, whereby todetermine an angular position of an object from which the transmittedsignal has been reflected.
 2. Apparatus according to claim 1 in whichthe truncated cross-correlation function is operable to shift one outputsignal with respect to another over a range which is less than theduration of the signals, and preferably less than the duration of apulse.
 3. Apparatus according to claim 1 or claim 2 in which thetruncated cross-correlation function is operable to shift one outputsignal with respect to another over a range in which the maximum offsetin either direction is less than 5 times the time that would be takenfor the transmitted signal to travel directly from one receiving elementto another, and preferably less than or equal to 3, 2 or 1 times thisvalue.
 4. Apparatus according to any of the preceding claims in whichthe signal has a characteristic wavelength λ and the truncatedcross-correlation function has an interval of correlation which is asmall multiple of λ.
 5. Apparatus according to claim 5 in which theinterval of correlation is less than 10, 5, or 2λ, or 1λ.
 6. Apparatusaccording to any preceding claim in which the receiving elements arespaced apart by a distance D and the truncated cross-correlationfunction has an interval that is less than a small multiple of D. 7.Apparatus according to claim 6 in which the interval of correlation isless than 5, 2, 1.5 or 1D, or, where the detection field of at least oneelement is less than 180 degrees, it may be 0.9D, 0.8D or 0.7D. 8.Apparatus according to any preceding claim in which the processing stageis operative to determine the distance from the array of an object fromwhich a signal has been reflected.
 9. Apparatus according to anypreceding claim in which the processing stage is operative to identify amaximum value of the cross correlation function.
 10. Apparatus forobtaining positional information relating to an object, preferablyaccording to any preceding claim, in which the processing stage isoperable to detect the interval between a signal being received by afirst set of any two or more of the receiving elements and to determinea first angular position of an object from which the transmitted signalhas been reflected; and to determine the interval between a signal beingreceived by a second set of any two or more of the receiving elementsand to determine a second angular position of an object from which thetransmitted signal has been reflected; the processing stage beingoperable by application of a truncated cross-correlation function. 11.Apparatus according to claim 10 in which the first and second angularpositions are measured in planes that are substantially not parallel toone another.
 12. Apparatus according to claim 11 in which the saidplanes are approximately normal to one another.
 13. Apparatus accordingto any preceding claim in which the processing stage is operative todetermine the distance from the array of an object from which a signalhas been reflected.
 14. Apparatus according to claim 13 in which theprocessing stage is operable to determine a coordinate inthree-dimensional space of an object from which the transmitted signalhas been reflected.
 15. Apparatus according to any one of claims 10 to14 in which at least one of the first and second set of receivingelements includes three or more elements which are disposed such thatthe spacings between elements of at least two pairs of elements includedin that set are unequal.
 16. Apparatus according to claim 12 in whichthe spacing (D) between the elements of one pair is approximately equalto (for example between 50% and 200% or between 75 and 150% of) acharacteristic wavelength λ of the signal, and preferably the spacingbetween a second pair of elements is approximately equal to 3λ/4, or 3D/4, and preferably the ratio of the spacing of the elements in one ofthe first and second pairs to the spacing of the elements in the otherof the first and second pairs is between 0.5 and 1 or 0.75 and 0.9. 17.Apparatus according to claim 10 or 16 in which the processing stage isoperative to perform a truncated cross-correlation between the signalsreceived by each pair of elements and to determine the product orotherwise compare of the result of the cross-correlations.
 18. Apparatusaccording to any preceding claim further comprising an output stageoperative to generate an output for presentation of positionalinformation relating to the object to a user.
 19. Apparatus according toclaim 18 in which the output includes at least one of an audible and avisual signal.
 20. Apparatus for obtaining positional informationrelating to an object, optionally in accordance with any precedingclaim, the apparatus comprising: a warning zone definition stage fordefining a warning zone (in two or three dimensions) within a detectionfield of the apparatus; and a discrimination stage for determiningwhether a detected object is within the warning zone; in which thewarning zone is defmed as a three-dimensional region within thedetection field.
 21. Apparatus according to claim 20 in which thewarning zone is contained within and is smaller than the detection fieldof the apparatus.
 22. Apparatus according to claim 20 or claim 21 inwhich the shape of the warning zone is dissimilar from the shape of thedetection field of the apparatus.
 23. Apparatus according to any one ofclaims 20 to 22 in which the shape of the warning zone is non-circularor non-spherical.
 24. Apparatus according to claim any one of claims 20to 23 in which the warning zone definition stage includes an algorithmthat defines a warning zone as a function of a coordinate within thedetection field.
 25. Apparatus according to any one of claims 20 to 24further comprising an object location stage for determining the positionof a detected object within the detection field of the apparatus. 26.Apparatus according to any one of claims 20 to 25 in which thediscrimination stage includes a coordinate generating stage forgenerating a coordinate of a detected object, which coordinate is thencompared with the warning zone.
 27. Apparatus according to claims 20 to24 in which the discrimination stage is operable to determine thecoordinates of the detected object and compare the determinedcoordinates with the coordinates of the warning zone to determinewhether the object is within the warning zone.
 28. Apparatus accordingto any one of claims 20 to 27 in which the warning zone definition stagedefines at least a limiting value of one or more ordinates of acoordinate within the detection field.
 29. Apparatus according to claim28 in which the warning zone definition stage defines at least alimiting value of one or more angles of a polar coordinate within thedetection field.
 30. Apparatus according to claim 29 in which thewarning zone definition stage defines at least a limiting value of arange of a polar coordinate within the detection field.
 31. Apparatusaccording to any one of claims 20 to 30 in which the warning zoneincludes a plurality of discontinuous spatial regions.
 32. Apparatusaccording to any one of claims 20 to 31 in which the discriminationstage is operative to generate an output signal indicative that theobject is within the warning zone.
 33. Apparatus according to any one ofclaims 20 to 32 further operative to issue a warning, for example atleast one of an audible, a visual and a tactile warning to a user upondetection of an object in the warning zone.
 34. Apparatus according toany one of claims 20 to 33 in which the warning zone definition stagedefines a plurality of non-coextensive warning zones, and preferably inwhich the discrimination stage is operative to generate an output signalindicative of which of the plurality of warning zones contains theobject.
 35. Apparatus according to claim 34 in which the discriminationstage is operable to apply different logic to at least two of the zones.36. Apparatus according to anyone of claims 20 to 35 in which thewarning zone is limited in range and/or is approximately cuboid. 37.Apparatus according to any of claims 20 to 36 in which thediscrimination stage is operative to analyse a characteristic of anobject outside of the warning zone.
 38. Apparatus according to claim 37in which the discrimination stage is operative to track an objectoutside the warning zone and to predict its entry into the warning zone.39. Apparatus according to any one of claims 20 to 38 for use on avehicle, and optionally including the vehicle.
 40. Apparatus accordingto claim 39 in which at least one of the shape and a relevant dimensionof a warning zone is at least in part determined by a correspondingshape and dimension of the vehicle.
 41. Apparatus according to claim 40in which a warning zone has a width that is equal to the width of thevehicle plus a predetermined amount.
 42. Apparatus according to claim 41in which the predetermined amount is less than the width of the vehicle.43. Apparatus according to any one of claims 39 to 42 in which thewarning zone has a height that is equal to the height of the vehicleplus a predetermined amount.
 44. Apparatus according to claim 43 inwhich the predetermined amount is less than the height of the vehicle.45. Apparatus according to any one of claims 39 to 44 in which thewarning zone has a lower surface that is substantially planar and spacedabove a road surface upon which the vehicle is supported.
 46. Apparatusaccording to any one of claims 39 to 45 in which at least one of theshape and a relevant dimension of the warning zone is changed inresponse to vehicle operating conditions.
 47. Apparatus according toclaim 46 in which the vehicle operating conditions include at least oneof speed, direction of travel, and ambient environmental conditions. 48.Apparatus according to any preceding claim operative to transmit anelectromagnetic signal.
 49. Apparatus according to claim 48 in which theelectromagnetic signal is microwave radiation.
 50. Apparatus accordingto claim 49 in which the transmitted signal has a frequency of between0.5 and 77 GHz.
 51. Apparatus according to claim 50 in which thetransmitted signal has a frequency of between 2 and 25 GHz. 52.Apparatus according to claim 51 in which the transmitted signal has afrequency of approximately one of 0.5 GHz, 1 GHz, 6 GHz, 10 GHz or 2-2.5GHz.
 53. Apparatus according to claim 52 in which the signal isapproximately 2.45 GHz.
 54. Apparatus according to any one of claims 48to 53 in which the transmitted signal has a relative bandwidth withrespect to the centre frequency of between 3 and 33%.
 55. Apparatusaccording to claim 54 in which the transmitted signal has a relativebandwidth with respect to the centre frequency of between 5 and 25%. 56.Apparatus according to claim 55 in which the transmitted signal has arelative bandwidth with respect to the centre frequency of between 10and 20%.
 57. Apparatus according to claim 56 in which the transmittedsignal has a relative bandwidth with respect to the centre frequency ofapproximately 15%.
 58. Apparatus for obtaining positional informationrelating to an object according to any preceding claim, which isoperative to transmit a signal into a detection field and to detect asignal reflected from an object in the detection field, in which thespatial length of the transmitted signal during its propagation isapproximately the same as a dimension of the smallest objects that theapparatus is intended to resolve.
 59. Apparatus for obtaining positionalinformation relating to an object according to claim 58 in which thespatial length of the transmitted signal during its propagation isbetween 50 and 200% of the smallest objects that the apparatus isintended to resolve.
 60. Apparatus for obtaining positional informationrelating to an object according to claim 58 in which the spatial lengthof the transmitted signal during its propagation is between 75 and 150%of the smallest objects that the apparatus is intended to resolve. 61.Apparatus according to any preceding claim in which the spatial lengthof the transmitted signal during its propagation is, in order ofmagnitude, not greater than 1.0 m, or preferably not greater than 0.3 m,0.1 m, 0.03 m, or even 0.01 m.
 62. Apparatus according to claim 48 orclaim 49 in which the spatial length of the transmitted signal duringits propagation is less than 10 wavelengths, or preferably less than 6,5, 3, 2 or even 1 wavelengths.
 63. Apparatus according to any precedingclaim in which the signal is received at the apparatus at receivingelements spaced by a distance being of the same order of magnitude as acharacteristic wavelength λ of the signal.
 64. Apparatus according toclaim 63 in which the receiving elements are spaced apart by a distancenλ where 0.5≦n≦10, preferably 1≦n≦5.
 65. Apparatus according to anypreceding claim, in which: the signal generating stage applies a seriesof m pulses to the transmitting element to cause it to transmit asignal, at times t_(n) where n=1, 2 . . . m, such that at least aportion of the signal can be reflected from the object to be received bythe receiving elements; the detection stage detects a signal reflectedto the receiving elements at times r_(n) and generates an output signalrepresentative of the received signal; wherein the value of r_(n)−t_(n)varies as some function of n.
 66. Apparatus according to claim 65 inwhich the value of r_(n)−t_(n) changes linearly with n.
 67. Apparatusaccording to claim 64 in which successive outputs of the detection stageare stored in a storage means, the storage means being operable tooutput a signal of substantially the same shape as the received signal,but with a duration that is increased in time.
 68. Apparatus accordingto any preceding claim including a sampling stage operative under thecontrol of a timing stage selectively to pass or to interrupt thepassage of signals from the receiving elements to the detection stage.69. Apparatus according to claim 68 in which, upon activation by thetiming stage, the sampling stage passes signals to the detection stagefor an aperture time ta.
 70. Apparatus according to claim 69 comprisinga respective sampling stage for each receiving element.
 71. Apparatusaccording to claim 70 in which each sampling stage is connected to thetiming stage by a respective signal delay line, within which delay linea signal is delayed by a time not less than t_(a)/2.
 72. Apparatus forobtaining positional information relating to an object according to anypreceding claim, contained within a single housing.
 73. Apparatusaccording to claim 72 that is hand-holdable in use.
 74. Apparatusaccording to claim 73 intended to provide information about the locationof objects within or behind a wall.
 75. Apparatus according to anypreceding claim comprising an antenna array and processing meansconstructed as a single assembly.
 76. Apparatus according to claim 75 inwhich the processing means operates to provide all functional electricalsignals to and receive all functional electrical signals from the array.77. Apparatus according to any one of claims 1 to 76 for use in avehicle.
 78. Apparatus for a land vehicle for obtaining positionalinformation relating to an object, according to any preceding claim,comprising: means for transmitting a probe signal towards the object;means for receiving, at a plurality of spaced apart locations, the probesignal as returned by the object; and detecting means, coupled to thereceiving means, for detecting the relative timing of the returned probesignals as received at the plurality of locations; whereby thepositional information for the object can be determined from saidrelative timing.
 79. Apparatus for obtaining positional informationrelating to an object, comprising: a transmitting stage for transmittinga probe signal towards the object, the transmitting stage comprising: asignal generating stage; and at least one transmitting element; areceiving stage for receiving, at a plurality of spaced apart locations,the probe signal as returned by the object, the receiving stagecomprising; at least one receiving element at the plurality of spacedapart locations; and a detecting stage for detecting the relative timingof the returned probe signals as received at the plurality of locations,the detecting stage comprising; a detection stage, coupled to thereceiving stage; whereby positional information for the object can bedetermined from the relative timing; and wherein: the signal generatingstage applies a series of m pulses to the transmitting element to causeit to transmit a signal, at times t_(n) where n=1, 2 . . . m, such thatat least a portion of the signal can be reflected from the object to bereceived by the receiving elements; the detection stage detects a signalreflected to the receiving elements at times r_(n) and generates anoutput signal representative of the received signal; and wherein thevalue of r_(n)−t_(n) varies as some function of n.
 80. Apparatus forobtaining positional information relating to an object, for use on avehicle, optionally in accordance with any preceding claim, forresolving the angular position of an object using non-Doppler radar. 81.Apparatus optionally according to any preceding claim, for use on avehicle, for obtaining positional information relating to an objectexternal of or internal to the vehicle, the apparatus being operative togenerate 3-dimensional positional data for the object.
 82. Apparatusaccording to any one of claims 77 to 81 in which the apparatus has adetection field within a passenger compartment of the vehicle. 83.Apparatus according to any one of claims 77 to 82 in which thepositional data includes at least one of the range, azimuth andelevation of the object.
 84. Apparatus according to any one of claims 77to 83 in which the antenna array is carried on a fixed location on thevehicle.
 85. Apparatus according to any one of claims 77 to 84 in whichthe antenna array is located within a component of the vehicle,preferably a non-metallic component.
 86. Apparatus according to any oneof claims 77 to 85 in which the antenna array is located within a bumperof the vehicle.
 87. Apparatus according to any one of claims 77 to 86 inwhich the antenna array is located within a non-metallic bumper. 88.Apparatus according to any one of claims 77 to 87 further comprisingalerting apparatus for alerting a vehicle driver to the presence of adetected object.
 89. Apparatus according to claim 88 in which thealerting apparatus is operative to generate an audible warning. 90.Apparatus according to claim 89 in which the audible warning includes averbal warning.
 91. Apparatus according to any one of claims 88 to 90 inwhich the alerting apparatus is operative to generate a visual warning.92. Apparatus according to claim 91 in which the visual warning includesa visible representation of the position of an object detected by theapparatus.
 93. Apparatus according to claim 92 further comprising adisplay upon which is presented a visual representation of a detectionfield of the apparatus and an object within the detection field. 94.Apparatus for obtaining positional information relating to an object,for use on a vehicle, optionally in accordance with any preceding claim,using preferably non-Doppler radar and being operative to determine aradar cross-section of an object.
 95. Apparatus for obtaining positionalinformation relating to an object, for use on a land vehicle optionallyin accordance with any preceding claim, including a transmitting elementfor transmitting radiation into a detection field, a receiving elementfor receiving radiation reflected from an object in the detection field,and a processing stage, which is operative to analyse the signals fromthe receiving element to derive qualitative information relating to theobject.
 96. Apparatus according to claim 95 in which the processingstage is operative to compare information relating to an object atsuccessive different angular positions against a look-up table. 97.Apparatus according to claim 95 or 96 in which the processing stage isoperative to determine a radar cross-section of the object. 98.Apparatus according to claim 97 in which the processing stage isoperative to compare the radar cross section with a threshold value ofradar cross-section and to generate a warning signal in dependence uponthe result of the comparison.
 99. Apparatus according to any one ofclaims 95 to 98 in which the processing stage is operative to determinean evolution of angular position of the object.
 100. Apparatus accordingto any of one claims 95 to 99 in which the processing stage is operativeto predict a path of movement of the object.
 101. Apparatus forobtaining positional information relating to an object substantially asherein described with reference to the accompanying drawings.
 102. Avehicle equipped with apparatus according to any preceding claim.
 103. Amotor road vehicle being equipped with a driver warning system, whichsystem comprises apparatus according to any one of claims 1 to 100 forobtaining positional information relating to an object external of thevehicle, the apparatus being operative to generate 3-dimensionalpositional data for the object.
 104. A road vehicle according to claim103 in which the array of the apparatus is contained within anon-metallic bumper of the vehicle.
 105. A road vehicle according toclaim 104 comprising a display instrument operative to processinformation obtained by the apparatus and to generate a displaytherefrom for an operator of the vehicle.
 106. A control system for airbags in a motor road vehicle, the control system comprising apparatusaccording to any one of claims 1 to 100 in which the apparatus has adetection field within a passenger compartment of the vehicle, and inwhich the processing stage is operational to determine the occupancy ofa seat equipped with a passenger air bag, and to suppress deployment ofthe bag in dependence on the occupancy of the seat, for example, if theseat is unoccupied, the occupant is too close to the air bag, or if theoccupant is determined to have violated a substantially planar surfaceor a substantially cuboid volume.
 107. A device for obtaininginformation about objects within or behind a wall comprising a apparatusaccording to any one of claims 1 to
 100. 108. A hand-held toolincorporating apparatus according to any one of claims 1 to
 100. 109.Apparatus for obtaining positional information relating to an object,comprising: means for transmitting a probe signal towards the object,said transmitting means comprising a transmitting element; means forreceiving, at a plurality of spaced apart locations, the probe signal asreturned by the object, said receiving means comprising a plurality ofreceiving elements; and detecting means, coupled to the receiving means,for detecting the relative timing of the returned probe signals asreceived at the plurality of spaced apart locations; whereby thepositional information for the object can be determined from saidrelative timing; and wherein the transmitting element and receivingelements are disposed on a common substrate.
 110. An electromagnetic(optionally microwave) antenna array optionally for use in apparatus forobtaining positional information relating to an object in accordancewith any one of claims 1 to 100, the array including a transmittingelement and a plurality of receiving elements, the transmitting andreceiving elements being disposed on a common substrate.
 111. Anelectromagnetic antenna array according to claim 109 or 110 including asingle transmitting element.
 112. An electromagnetic antenna arrayaccording to any one of claims 109 or 111 including three receivingelements arranged non-collinearly.
 113. An electromagnetic antenna arrayoptionally for use in apparatus for obtaining positional informationrelating to an object, the array including a transmitting element and atleast three receiving elements arranged non-collinearly, thetransmitting and receiving elements being disposed on a commonsubstrate.
 114. An electromagnetic antenna array according to claims 112or 113 in which the receiving elements are arranged substantially at thevertices of a right-angled triangular locus.
 115. An electromagneticantenna array according to any one of claims 109 to 114 in which thereceiving elements are spaced apart by a distance that is the same orderof magnitude as the wavelength λ of the radiation that it is intended totransmit and receive.
 116. An electromagnetic antenna array according toclaim 115 in which the receiving elements are spaced apart by a distancemλ where m is less than 10, and preferably less than 8, 5, 3, or 2, andm is greater than 0.1 and preferably greater than 0.2, 0.3 or 0.5. 117.An electromagnetic antenna array according to claim 115 or 116 includingfour receiving elements arranged non-collinearly.
 118. Anelectromagnetic antenna array according to any one of claims 115 to 117including at least three receiving elements arranged non-collinearlysuch that there is an axis about which the array is asymmetrical. 119.An electromagnetic antenna array including at least three receivingelements arranged non-collinearly such that there is an axis about whichthe array is asymmetrical.
 120. An electromagnetic antenna arrayaccording to any of claims 109 to 119 in which the receiving elementsare arranged substantially at the vertices of a quadrilateral.
 121. Anelectromagnetic antenna array according to any of claims 109 to 120 inwhich the receiving elements are arranged substantially at the verticesof a trapezial locus.
 122. An electromagnetic antenna array according toany of claims 109 to 121 in which the trapezial locus is rectangular.123. An electromagnetic antenna array according to any of claims 109 to121 in which the trapezial locus has long and short parallel sides andpreferably the short side is between 0.5 and 1 times the length of thelong side, or in which the quadrilateral has two opposing angles whichare substantially right angles, the other two angles not being rightangles.
 124. An electromagnetic antenna array according to any of claims109 to 122 in which the trapezial locus has long and short parallelsides, the length of the shorter side being approximately the wavelengthλ of the radiation that the array is intended to transmit and receive,and the length of the longer side is approximately 3λ/2.
 125. Anelectromagnetic antenna array according to any one of claims 109 to 124for use in apparatus according to any one of claims 1 to
 100. 126. Anelectromagnetic antenna array optionally in accordance with any one ofclaims 109 to 125, for use in apparatus for obtaining positionalinformation relating to an object, which apparatus may be in accordancewith any preceding claim, the array including a transmitting element anda plurality of receiving elements, in which the spacing of two pairs ofthe receiving elements in a common direction is unequal.
 127. Apparatusfor obtaining positional information relating to one or more objects,the apparatus being operative in an operating cycle for each of m stepsin which n=1, 2 . . . m, the apparatus including: a signal generatingstage operative, simultaneously with or at a fixed time after atransmitting trigger instant t_(n) to generate a signal, and atransmitting element to transmit said signal into a detection field; aplurality of spaced receiving elements operative simultaneously with orat a fixed time after a receiving trigger instant r_(n) to receive aportion of the signal reflected from one or more objects in thedetection field, the interval r_(n)−t_(n) varying as a function of n andhaving a magnitude in a range corresponding to the times of travel of asignal reflected from an object within the detection field; means foridentifying the values of n at which signals reflected from one objectare received at two or more receiving elements and thereby detecting thetime taken, and therefore the distance travelled, by the signals fromthe transmitting element to the various receiving elements; and meansfor calculating the position of the object from the various path lengthsthereby identified.
 128. Apparatus for generating an image of objectswithin or through a solid object according to any one of claims 1 to101.
 129. Apparatus according to claim 128 in which the solid object isa wall.
 130. Apparatus according to any one of claims 1 to 101 forproviding an image of an environment in conditions that human vision iscompromised.
 131. Apparatus according to claim 130 in which vision iscompromised by the physiological condition of a user.
 132. Apparatusaccording to claim 130 or claim 131 in which vision is compromised byenvironmental conditions.
 133. A method for obtaining positionalinformation relating to an object, optionally in apparatus in accordancewith any one of claims 1 to 100, comprising: applying a series of pulsesto a transmitting element of an (optionally fixed) array to cause it totransmit a signal, such that at least a portion of the signal isreflected from the object to be received by the receiving elements;detecting signals reflected to receiving elements of the array andgenerating output signals representative of the received signals; andapplying a truncated cross-correlation function to the output signals todetect the interval between signals received by a plurality of thereceiving elements, whereby to determine an angular position of anobject from which the transmitted signal has been reflected.
 134. Amethod according to claim 133 in which the truncated cross-correlationfunction comprises shifting one output signal with respect to anotherover a range which is less than the duration of the signals, andpreferably less than the duration of a pulse.
 135. A method according toclaim 133 or 134 in which the truncated cross-correlation functioncomprises shifting one output signal with respect to another over arange in which the maximum offset in either direction is less than 5times the time that would be taken for the transmitted signal to traveldirectly from one receiving element to another, and preferably less thanor equal to 3, 2 or 1 times this value.
 136. A method according to anyof the preceding method claims in which the signal has a characteristicwavelength λ and the truncated cross-correlation function has aninterval of correlation which is a small multiple of λ.
 137. A methodaccording to claim 136 in which the interval of correlation is less than10, 5, or 2λ, or even λ.
 138. A method according to any of the precedingmethod claims in which the receiving elements are spaced apart by adistance D and the truncated cross-correlation function has an intervalthat is less than a small multiple of D.
 139. A method according toclaim 138 in which the interval of correlation is less than 5, 2, 1.5 or1D, or, where the detection field of at least one element is less than180 degrees, it may be 0.9D, 0.8D or 0.7D.
 140. A method according toany of the preceding method claims further including determining thedistance from the array of an object from which a signal has beenreflected.
 141. A method according to claim 140 in which determining thedistance from the array includes a step of multiplying the time takenfor the signal to be received by the speed of propagation of the signal.142. A method according to any of the preceding method claims in which amaximum value of the cross correlation function is identified.
 143. Amethod according to any one of claims 133 to 142 including the steps of:determining the interval between a signal being received by a first setof any two or more of the receiving elements; calculating a firstangular position of an object from which the transmitted signal has beenreflected; determining the interval between a signal being received by asecond set of any two or more of the receiving elements; and calculatinga second angular position of an object from which the transmitted signalhas been reflected.
 144. A method according to claim 143 in which thefirst and second angular positions are measured in planes that aresubstantially not parallel to one another.
 145. A method according toclaim 144 in which the said planes are approximately normal to oneanother.
 146. A method according to claim 145 including determining acoordinate in three-dimensional space of an object from which thetransmitted signal has been reflected.
 147. A method according to anyone of claims 143 to 146 in which at least one of the first and secondset of receiving elements includes three or more elements which aredisposed such that that set includes at least two pairs of elements, thespacing of elements in the two pairs being unequal.
 148. A methodaccording to claim 147 in which the spacing (D) between the elements ofone pair is approximately equal to (for example between 50% and 200% orbetween 75 and 150% of) a characteristic wavelength λ of the signal, andpreferably the spacing between a second pair of elements isapproximately equal to 3λ/4, or 3 D/4, and preferably the ratio of thespacing of the elements in one of the first and second pairs to thespacing of the elements in the other of the first and second pairs isbetween 0.5 and 1 or 0.75 and 0.9.
 149. A method according to claim 147or 148 in which the truncated cross-correlation is performed between thesignals received by each pair of elements and the product or anothercomparison of the result of the cross-correlations is determined.
 150. Amethod according to any preceding method claim further comprisinggenerating an output for presentation of positional information relatingto the object to a user.
 151. A method according to claim 150 in whichthe output includes at least one of an audible and a visual signal. 152.A method for obtaining positional information relating to an object,optionally in accordance with any preceding method claim, comprising:defining a warning zone (in two or three dimenstions) within a detectionfield; and determining whether a detected object is within the warningzone: wherein the warning zone is defined as a three-dimensional regionwithin the detection field.
 153. A method according to claim 152 inwhich the warning zone is contained within and is smaller than thedetection field.
 154. A method according to claim 152 or 153 in whichthe shape of the warning zone is dissimilar from the shape of thedetection field.
 155. A method according to any one of claims 152 to 154in which the warning zone includes a region defined in two dimensionswithin the detection field, preferably in which the warning zoneincludes a planar surface within the detection field.
 156. A methodaccording to any one of claims 152 to 155 in which the warning zone isdefined as a three-dimensional region within the detection field.
 157. Amethod according to any one of claims 152 to 156 in which the warningzone is defined by an algorithm as a function of a coordinate within thedetection field.
 158. A method according to any one of claims 152 to 157further including generation of a co-ordinate of a detected object. 159.A method according to claim 158 in which the generated co-ordinates arecompared with co-ordinates of the warning zone to determine whether theobject is within the warning zone.
 160. A method according to any one ofclaims 152 to 159 in which the warning zone is defined by at least alimiting value of one or more ordinates of a coordinate within thedetection field.
 161. A method according to claim 160 in which thewarning zone is defined by at least a limiting value of one or moreangles of a polar coordinate within the detection field.
 162. A methodaccording to any one of claims 152 to 161 in which the warning zone isdefined by at least a limiting value of a range of a polar coordinatewithin the detection field.
 163. A method according to any one of claims152 to 162 in which the warning zone includes a plurality ofdiscontinuous spatial regions.
 164. A method according to any one ofclaims 152 to 163 further comprising the step of generating an outputsignal indicative that the object is within the warning zone.
 165. Amethod according to any one of claims 152 to 164 further comprising thestep of issuing a warning to a user upon detection of an object in thewarning zone.
 166. A method according to any one of claims 152 to 165 inwhich there is defined a plurality of non-coextensive warning zones 167.A method according to claim 166 which includes a step of generating anoutput signal indicative of which of the plurality of warning zonescontains the object.
 168. A method according to claim 166 or claim 167including a step of applying different logic to at least two of thezones.
 169. A method according to any of claims 152 to 168 furthercomprising the step of analysing a characteristic of an object outsideof the warning zone.
 170. Apparatus according to claim 169 in which thestep of analysing a characteristic comprises tracking an object outsidethe warning zone and predicting its entry into the warning zone.
 171. Amethod according to any one of claims 152 to 170 further comprising thestep of issuing at least one of an audible and a visual warning to auser upon detection of an object in a warning zone.
 172. A methodaccording to any one of claims 152 to 171 carried out on a vehicle, inwhich at least one of the shape and a relevant dimension of a warningzone is at least in part determined by a corresponding shape anddimension of the vehicle.
 173. A method according to claim 172 whichincludes monitoring operating conditions of the vehicle and changing atleast one of the shape and a relevant dimension of a warning zone inresponse to vehicle operating conditions.
 174. A method according toclaim 173 in which the vehicle operating conditions include at least oneof speed, direction of travel, and ambient environmental conditions.175. A method according to 173 or claim 174 in which the distance towhich the warning zone extends in the direction of travel of the vehicleis increased with the speed of the vehicle.
 176. A method according toany one of claims 173 to 175 in which the extent to which the warningzone extends to one side of a longitudinal axis of the vehicle isincreased in a direction in which the vehicle is turning.
 177. A methodaccording to any one of the preceding method claims in which the signalis an electromagnetic signal.
 178. A method according to claim 177 inwhich the electromagnetic signal is microwave radiation.
 179. A methodaccording to claim 178 in which the electromagnetic signal has afrequency of between 0.5 and 77 GHz.
 180. A method according to claim179 in which the electromagnetic signal has a frequency of between 2 and25 GHz.
 181. A method according to claim 179 in which theelectromagnetic signal has a frequency of approximately one of 0.5 GHz,1 GHz, 6 GHz, 10 GHz or 2-2.5 GHz.
 182. A method according to claim 181in which the electromagnetic signal has a frequency of approximately2.45 GHz.
 183. A method according to any one of claims 177 to 182 inwhich the transmitted signal has a relative bandwidth to the centrefrequency between 10 and 20%.
 184. A method according to claim 183 inwhich the transmitted signal has a relative bandwidth to the centrefrequency between 5 and 25%.
 185. A method according to claim 184 inwhich the transmitted signal has a relative bandwidth to the centrefrequency between 3 and 33%.
 186. A method according to claim 185 inwhich the transmitted signal has a relative bandwidth to the centrefrequency of approximately 15%.
 187. A method according to any one ofclaims 180 to 186 in which an angular position of an object is resolvedwith respect to a predetermined datum.
 188. A method for obtainingpositional information relating to an object, optionally according toany one of claims 142 to 187 comprising transmitting a signal into adetection field and detecting a signal reflected from an object in thedetection field, in which the spatial length of the transmitted signalduring its propagation is approximately the same as (say between 50 and200% or 75 and 150% of) a dimension of the smallest objects that it isintended to resolve.
 189. A method according to any preceding methodclaim in which the spatial length of the transmitted signal during itspropagation is, in order of magnitude, not greater than 1.0 m, andpreferably not greater than 0.3 m, 0.1 m, 0.03 m, or even 0.01 m.
 190. Amethod according to claim 188 or 189 in which the spatial length of thetransmitted signal during its propagation is less than 10 wavelengths,or less than 6, 5, 3, 2 or even 1 wavelengths.
 191. A method accordingto any preceding method claim in which the signal is received atreceiving elements spaced by a distance being of the same order ofmagnitude as a characteristic wavelength λ of the signal.
 192. A methodaccording to claim 191 in which the receiving elements are spaced apartby a distance nλ where 0.5≦n≦10, and preferably 1≦n≦5.
 193. A methodaccording to any preceding method claim in which a series of pulsescomprises m pulses to cause the transmitting element to transmit asignal, at times t_(n) where n=1, 2 . . . m; and reflected signals aredetected by the receiving elements at times r_(n); comprising the stepsof generating an output signal representative of the received signal;wherein the value of r_(n)−t_(n) varies as some function of n.
 194. Amethod according to claim 193 in which the value of r_(n)−t_(n) changeslinearly with n.
 195. A method according to claim 193 or 194 furthercomprising storing values of the output signal corresponding to signalsreceived at times r_(n).
 196. A method according to claim 195 furthercomprising outputting a signal of substantially the same shape as thereceived signal, but with a duration which is increased in time.
 197. Amethod according to any preceding method claim for obtaining positionalinformation relating to an object, performed on a vehicle.
 198. A methodaccording to claim 197 for resolving the angular position of an objectusing non-Doppler radar.
 199. A method according to claim 197 or 198 forobtaining positional information relating to an object external of thevehicle, in which 3-dimensional positional data for the object isgenerated.
 200. A method according to any one of claims 197 to 199 forobtaining positional information relating to an object internal to thevehicle, in which 3-dimensional positional data for the object isgenerated.
 201. A method according to any one of claims 197 to 200 inwhich a detection field is within a passenger compartment of thevehicle.
 202. A method according to any one of claims 197 to 201 inwhich the positional data includes at least one of the range, azimuthand elevation of the object.
 203. A method according to any one ofclaims 197 to 202 which is carried out by apparatus including an antennaarray that is carried on a fixed location on the vehicle.
 204. A methodaccording to any one of claims 197 to 203 in which the antenna array islocated within a non-metallic component of the vehicle.
 205. A methodaccording to any one of claims 197 to 204 in which the antenna array islocated within a non-metallic bumper of the vehicle.
 206. A methodaccording to any one of claims 197 to 205 further comprising a step ofalerting a vehicle driver to the presence of a detected object.
 207. Amethod according to claim 206 in which the alerting step includesgenerating an audible warning.
 208. A method according to claim 207 inwhich the audible warning includes a descriptive verbal warning.
 209. Amethod according to any one of claims 206 to 208 in which the alertingstep includes generating a visual warning.
 210. A method according toclaim 209 in which the visual warning includes an image of the positionof a detected object.
 211. A method according to claim 209 furthercomprising a step of presenting a visual representation of a detectionfield and an object within the detection field.
 212. A method forobtaining positional information relating to an object, performed on avehicle optionally in accordance with any preceding method claim usingnon-Doppler radar, the method including determining a radarcross-section of an object.
 213. A method for obtaining positionalinformation relating to an object, in accordance with any precedingmethod claim, including transmitting radiation into a detection field,receiving radiation reflected from an object in the detection field, andanalysing received signals to derive qualitative information relating tothe object.
 214. A method according to claim 213 in which a radarcross-section of the object is determined.
 215. A method according toclaim 213 or 214 in which the radar cross section is compared with athreshold value of radar cross-section and a warning signal is issued independence upon the result of the comparison.
 216. A method according toany one of claims 213 to 215 in which an evolution of angular positionof the object is determined.
 217. A method according to any one ofclaims 213 to 216 in which a path of movement of the object ispredicted.
 218. A method according to any one of claims 213 to 217 inwhich in the analysis step the signals are modified to compensate forangular variation in sensitivity of the receiving element.
 219. A methodaccording to any one of claims 213 to 218 in which in the analysis stepthe signals are modified to compensate for the range of the object fromwhich the signals are reflected.
 220. A method according to any one ofclaims 213 to 219 in which the analysis step includes making acomparison between a received signal and a pattern corresponding to asignal from a known class of objects.
 221. A method according to claim220 in which the comparison includes identification of characteristicfeatures of the received signal.
 222. A method according to claim 221 inwhich the characteristic features include at least one of minima,maxima, and zero-crossings.
 223. A method for obtaining positionalinformation relating to an object substantially as herein described withreference to the accompanying drawings.
 224. A method of controllingdeployment of air bags in a vehicle, in which a method according to anypreceding method claim is applied to determine the occupancy of a seatequipped with a passenger air bag, and deployment of the bag issuppressed in dependence on the occupancy of the seat, for example, ifthe seat is unoccupied or the occupant is too close to the air bag. 225.A method of obtaining positional information relating to an object inapparatus in accordance with any one of claims 1 to 100, the methodcomprising an operating cycle having m steps in which n=1, . . . m, eachstep comprising: (a) generating a signal at a given timing relationshipwith respect to a transmitting trigger instant t_(n) and transmitting itinto a detection field; and (b) receiving at a given timing relationshipwith respect to a receiving trigger instant r_(n) at least a portion ofthe signal reflected from the object; wherein the interval r_(n)−t_(n)varies as a function of n.
 226. A method according to claim 225 in whichthe method further comprises: c) providing at least two spaced receivingelements and identifying the values of n at which signals reflected fromthe object are received by the receiving elements.
 227. A methodaccording to claim 226 including in step c) a comparison of theamplitude of signals received by various of the receiving elements. 228.A method according to any one of claims 225 to 227 in which step c)includes comparison of characteristic features of the received signals,such features including at least one of zero-crossings, maxima andminima.
 229. A method according to any one of claims 225 to 228 in whicha truncated correlation is carried out in step c), and in which thecorrelation coefficient is approximately a cosinusoidal function of eachoffset angle (θ-θ₀), where θ₀ is the angle offset of the object in theplane containing the relevant antenna pair in which the correlationcoefficients are determined at a small sample of angles, separated byless than half the sinusoidal wavelength, to estimate the direction ofthe maximum, followed by further samples close to that direction torefine the estimate.
 230. A method according to any one of claims 225 to229 including a step of selecting reflections denoted by correlationmaxima which exceed a predetermined signal threshold, and dividing eachsuch correlation maximum by the gain of the transmitting and receivingantennas in the measured direction.
 231. A method according to claim 230in which each such correlation maximum is multiplied by the fourth powerof the measured range, to obtain a value proportional to the radarcross-section of the object.
 232. A method according to claim 231 inwhich the said value is compared to a cross-section threshold such thatobjects whose cross-section exceeds such threshold are subjected tofurther processing.
 233. A method according to any one of claims 225 to232 in which the range of values of r_(n)−t_(n) for 1≦n≦m cover a rangeof time within which it is expected that a signal reflected from theobject will be received.
 234. A method according to any one of thepreceding method claims for providing an image of an environment inconditions that human vision is compromised.
 235. A method according toclaim 234 in which vision is compromised by the physiological conditionof a user.
 236. A method according to claim 234 or claim 235 in whichvision is compromised by environmental conditions.
 237. A methodaccording to any preceding method claim for imaging within or through asolid object.
 238. A method according to claim 237 in which the solidobject is a wall.
 239. Apparatus for obtaining positional informationrelating to an object, comprising: transmitting means for transmitting aprobe signal towards the object, the transmitting means comprising: asignal generating stage; and at least one transmitting element;receiving means for receiving, at a plurality of spaced apart locations,the probe signal as returned by the object, the receiving meanscomprising; at least one receiving element at the plurality of spacedapart locations; and detecting means for detecting the relative timingof the returned probe signals as received at the plurality of locations,the detecting means comprising; a detection stage, coupled to thereceiving means,; whereby positional information for the object can bedetermined from the relative timing; and wherein: the signal generatingstage applies a series of m pulses to the transmitting element to causeit to transmit a signal, at times t_(n) where n=1, 2 . . . m, such thatat least a portion of the signal can be reflected from the object to bereceived by the receiving elements; the detection stage detects a signalreflected to the receiving elements at times r_(n) and generates anoutput signal representative of the received signal; and wherein thevalue of r_(n)−t_(n) varies as some function of n.
 240. Apparatus forobtaining positional information relating to an object, comprising:means for transmitting a probe signal towards the object, saidtransmitting means comprising a transmitting element; means forreceiving, at a plurality of spaced apart locations, the probe signal asreturned by the object, said receiving means comprising a plurality ofreceiving elements; and detecting means, coupled to the receiving means,for detecting the relative timing of the returned probe signals asreceived at the plurality of spaced apart locations; whereby thepositional information for the object can be determined from saidrelative timing; and wherein the transmitting element and receivingelements are disposed on a common substrate.
 241. An antenna arrayoptionally for use in apparatus for obtaining positional informationrelating to an object in accordance with any one of claims 1 to 100, thearray including a transmitting element and a plurality of receivingelements, the transmitting and receiving elements being disposed on acommon substrate.
 242. A vehicle substantially as herein described andwith reference to the accompanying drawings
 243. A control system forair bags in a motor road vehicle substantially as herein described andwith reference to the accompanying drawings.
 244. Apparatus for a landvehicle substantially as herein described and with reference to theaccompanying drawings.
 245. A device for obtaining information aboutobjects within or through a wall substantially as herein described andwith reference to the accompanying drawings.
 246. A hand-held toolsubstantially as herein described and with reference to the accompanyingdrawings.
 247. An electromagnetic microwave antenna array substantiallyas herein described and with reference to the accompanying drawings.248. Apparatus for generating an image of objects within or through asolid object substantially as herein described and with reference to theaccompanying drawings.
 249. Apparatus, optionally for use on a (forexample, land) vehicle, for obtaining positional information relating toan object substantially as herein described with reference to theaccompanying drawings.
 250. A method for controlling deployment of airbags in a vehicle substantially as herein described.
 251. Method ofobtaining positional information relating to an object, comprising thesteps of: transmitting a probe signal towards the object; receiving, ata plurality of spaced apart locations, the probe signal as returned bythe object; detecting the relative timing of the returned probe signalsas received at the plurality of locations; and determining positionalinformation for the object from the relative timing; wherein: thetransmitting step comprises applying a series of m pulses to atransmitting element to cause it to transmit a signal, at times t_(n)where n=1, 2 . . . m, such that at least a portion of the signal can bereflected from the object to be received at the plurality of spacedapart locations; the detecting step comprises detecting a signalreflected to the receiving elements at times r_(n) and generating anoutput signal representative of the received signal; and wherein thevalue of r_(n)−t_(n) varies as some function of n.
 252. Method ofobtaining positional information relating to an object using anapparatus comprising a transmitting element, a receiving meanscomprising a plurality of receiving elements and a detecting means, themethod comprising: transmitting a probe signal from the transmittingelement towards the object; receiving, at a plurality of spaced apartlocations, the probe signal as returned by the object; and detecting, atthe detecting means, the relative timing of the returned probe signalsas received at the plurality of spaced apart locations; determining thepositional information for the object from said relative timing; whereinthe detecting means is coupled to the receiving means and thetransmitting element and receiving elements are disposed on a commonsubstrate.
 253. Use of an electromagnetic antenna array in a methodaccording to claim 252 in which the receiving elements are spaced apartby a distance that is the same order of magnitude as the wavelength λ ofthe radiation that it is intended to transmit and receive, theelectromagnetic antenna array including at least three receivingelements arranged non-collinearly such that there is an axis about whichthe array is asymmetrical.
 254. Use of an electromagnetic antenna arrayin a method of obtaining positional information relating to an object,the array including a transmitting element and at least three receivingelements arranged non-collinearly, the transmitting and receivingelements being disposed on a common substrate.
 255. Use of anelectromagnetic antenna array in a method of obtaining positionalinformation relating to an object, the array including at least threereceiving elements arranged non-collinearly such that there is an axisabout which the array is asymmetrical.